Britannica Illustrated Science Library

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For citation purposes, use the page numbers that appear in the text. UNIVERSEUNIVERSE

Encyclopædia Britannica, Inc. Chicago ■ London ■ New Delhi ■ Paris ■ Seoul ■ Sydney ■ Taipei ■ Tokyo Britannica Illustrated Science Library

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Editorial Jacob E. Safra, Chairman of the Board Michael Levy, Executive Editor, Core Editorial John Rafferty, Associate Editor, Earth Sciences Jorge Aguilar-Cauz, President William L. Hosch, Associate Editor, Mathematics and Computers Michael Ross, Senior Vice President, Corporate Development Kara Rogers, Associate Editor, Life Sciences Rob Curley, Senior Editor, Science and Technology Dale H. Hoiberg, Senior Vice President and Editor David Hayes, Special Projects Editor Marsha Mackenzie, Director of Production Art and Composition Steven N. Kapusta, Director Carol A. Gaines, Composition Supervisor Christine McCabe, Senior Illustrator International Standard Book Number (set): Media Acquisition 978-1-59339-797-5 Kathy Nakamura, Manager International Standard Book Number (volume): 978-1-59339-798-2 Copy Department Britannica Illustrated Science Library: Universe 2008 Sylvia Wallace, Director Julian Ronning, Supervisor Printed in China

Information Management and Retrieval Sheila Vasich, Information Architect

Production Control Marilyn L. Barton

Manufacturing Kim Gerber, Director www.britannica.com Universe PICTURE ON PAGE 1 Image of a planetary nebula. Contents Planetary nebulae are among the most photogenic objects in astronomy.

What Is the Universe?

Page 6

What Is in the Universe?

Page 18

The Solar System

Page 38

The Earth and the Moon

Page 66

Observing the Universe

Page 80 CONE NEBULA This nebula got its name from its cone shape, as shown in the image.

compare the destiny of other worlds similar the first time. Less than a decade ago, The Secrets of to ours will help us understand that for the astronomers began observing frozen worlds, time being there is no better place than the much smaller than a planet, in a region of Earth to live. At least for now. the solar system called the Kuiper belt. We the Universe invite you to explore all of this. The images n the Milky Way—according to and illustrations that accompany the text mathematical and physical will prove very helpful in studying and here was a time when people believed I calculations—there are more than 100 understanding the structure of all the visible that the stars were bonfires lit by billion stars, and such a multitude leads to and invisible objects (such as dark matter) T other tribes in the sky, that the the question: Is it possible that our Sun is that form part of the universe. There are universe was a flat plate resting on the shell the only star that possesses an inhabited stellar maps showing the constellations, the of a giant turtle, and that the Earth, planet? Astronomers are more convinced groups of stars that since ancient times have according to the Greek astronomer Ptolemy, than ever of the possibility of life in other served as a guide for navigation and for the was at the center of the universe. From the worlds. We just need to find them. Reading development of calendars. There is also a most remote of times, people have been this book will let you become better review through history: from Ptolemy, who curious about what lies hidden beyond the acquainted with our neighbors in the solar thought the planets orbited around the celestial sphere. This curiosity has led them system—the other planets—and the most Earth, and Copernicus, who put the Sun in to build telescopes that show with clarity important characteristics that distinguish the center, and Galileo, the first to aim a otherwise blurry and distant objects. In this them. All this information that explores the telescope skyward, up to the most recent book you will find the history of the cosmos mysteries of space is accompanied by astronomical theories, such as those of illustrated with spectacular images that recent images captured by the newest Stephen Hawking, the genius of space and show in detail how the cosmos was formed, telescopes. They reveal many details about time who continues to amaze with his the nature of the many points of light that the planets and their satellites, such as the discoveries about the greatest mysteries of adorn the night sky, and what lies ahead. volcanoes and craters found on the surface the cosmos. You will find these and many You will also discover how the suns that of some of them. You will also learn more more topics no matter where you look in this inhabit space live and die, what dark matter about the asteroids and comets that orbit fantastic book that puts the universe and its and black holes are, and what our place is in the Sun and about Pluto, a dwarf planet, secrets in your hands. this vastness. Certainly, the opportunity to which is to be visited by a space probe for DARK MATTER X-RAY OF THE COSMOS 8-9 Evidence exists that dark matter, though invisible to telescopes, betrays itself by the gravitational THE INSTANT OF CREATION 10-13 pull it exerts over other heavenly bodies. What Is the Universe? EVERYTHING COMES TO AN END 14-15 THE FORCES OF THE UNIVERSE 16-17

he universe is everything that visible and invisible things, such as dark literally determine the density of all of are asking—the question that concerns exists, from the smallest matter, the great, secret component of space, as well as decide the destiny of them the most—is how much longer the particles to the largest ones, the cosmos. The search for dark matter the universe. Did you know that, second universe can continue to expand like a T together with all matter and is currently one of the most important by second, the universe grows and balloon before turning into something energy. The universe includes tasks of cosmology. Dark matter may grows? The question that astronomers cold and dark. 8 WHAT IS THE UNIVERSE? UNIVERSE 9

Corona Borealis Supercluster

Capricornus Supercluster SUPERCLUSTERS. Within a FILAMENTS. From five billion X-Ray of the Cosmos distance of a billion light-years, light-years away, the immensity of 6. groups of millions of galaxies, 7. the cosmos is evident in its called superclusters, can be seen. galactic filaments, each one home he universe, marvelous in its majesty, is an ensemble of a hundred to millions and millions of galaxies. Hercules billion galaxies. Each of these galaxies (which tend to be found in Supercluster Boötes Void Boötes T large groups) has billions of stars. These galactic concentrations Supercluster 180° Pavo-Indus surround empty spaces, called cosmic voids. The immensity of the Supercluster cosmos can be better grasped by realizing that the size of our fragile planet Earth, or even that of the Milky Way, is insignificant Shapley Centaurus Supercluster compared to the size of the remainder of the cosmos. Supercluster Sculptor Sculptor Supercluster Void VIRGO Coma Ursa Major Supercluster Supercluster 250 Pisces-Cetus Superclusters Hydra

Leo 750 Supercluster 1,000 Pisces-Perseus 1. Supercluster Virgo III 0° Group EARTH Originated, together with the solar system, when The Universe the universe was already 9.1 Originating nearly 14 billion years ago Sextans billion years old. It is the only Supercluster in an immense explosion, the universe known planet that is home to life. today is too large to be able to conceive. The innumerable stars and galaxies that populate it 180° promise to continue expanding for a long time. Horologium EARTH Though it might sound strange today, for many Superclusters Columba Supercluster Pluto years, astronomers thought that the Milky Way, NGC Neptune NGC where the Earth is located, constituted the entire 6744 LOCAL 7582 GROUP NGC universe. Only recently—in the 20th century—was outer Jupiter NGC 5033 Uranus Virgo NGC space recognized as not only much vaster than previously 5128 M101 Canis Group 4697 Saturn thought but also as being in a state of ongoing expansion. Sculptor 12.5 Maffei 25 M81 37.5 Ursa Major NGC Leo I Group 50 1023 NGC 2997 Dorado 0° Fornax NEAR STARS Found closer Cluster than 20 light-years from the Eridanus Sextans B Leo III Sun, they make up our solar Cluster Group 2. Sextans A neighborhood. Ross Lalande G51-15 128 21185 Struve At a scale Wolf NEAREST GALAXIES. 2398 359 of one hundred million light-years, the galactic clusters nearest to 12.5 Leo A 5. Procyon Within a space 90° NEIGHBORS the Milky Way can be seen. Luyten’s 7.5 of one million light-years, NGC Star Bernard’s 2.5 we find the Milky Way and 3109 Antila Star 3. SUN 61 Cygni its closest galaxies. Dwarf 180° Alpha Centauri Ross Sirius 248 Sextans 270° Leo I Groombridge 0° Dwarf Leo II 34 Ross Ursa MILKY WAY 154 L789-6 Minor Dwarf Draco IC 10 180° Dwarf 1.2 NGC NGC 2.5 Epsilon 185 147 Eridani 3.7 M110 Andromeda I L726-8 Andromeda MILKY WAY 4. NGC M32 L789-6 6822 100 billion L372-58 Epsilon Canis Triangle Lacaille 0.12 Indi 0.25 Major LOCAL GROUP. Ten 0° The total number of galaxies that exist, Ceti 9352 Phoenix L725-32 0.37 Sagittarius million light-years away LGS 3 Dwarf indicating that the universe is both larger 0.5 Large Dwarf is Andromeda, the Magellanic IC Aquarius and older than was previously thought Pegasus Cloud closest to the Earth. 1613 Dwarf Dwarf Carina Cetus Dwarf 0° Dwarf Sagittarius Small Tucana Irregular Magellanic Dwarf Dwarf Cloud WLM 10 WHAT IS THE UNIVERSE? UNIVERSE 11

WMAP (WILKINSON MICROWAVE ANISOTROPY PROBE) Cosmic Inflation Theory NASA's WMAP project maps the background radiation of the universe. In the The Instant of Creation Although big bang theorists understood the universe as originating image, hotter (red-yellow) regions and colder (blue-green) regions can be in an extremely small, hot, and condensed ball, they could not observed. WMAP makes it possible to determine the amount of dark matter. t is impossible to know precisely how, out of nothing, the universe began to exist. According to the big understand the reason for its staggering growth. In 1981, physicist Alan Guth proposed a solution to the problem with his inflationary theory. In an bang theory—the theory most widely accepted in the scientific community—in the beginning, there extremely short period of time (less than a thousandth of a second), the I appeared an infinitely small and dense burning ball that gave rise to space, matter, and energy. This universe grew more than a trillion trillion trillion times. Near the end of this happened 13.7 billion years ago. The great, unanswered question is what caused a small dot of light—filled period of expansion, the temperature approached absolute zero. with concentrated energy from which matter and antimatter were created—to arise from nothingness. In very little time, the young universe began to expand and cool. Several billion years later, it acquired the form we know today.

Galaxy 1 Galaxy 2 Region 1 Region 3

HOW IT GREW HOW IT DID THE SEPARATION OF FORCES Cosmic inflation was NOT GROW Before the universe expanded, during a period of Gravity an expansion of the Had the universe not radiation, only one unified force governed all Galaxy 3 entire universe. The Galaxy 4 undergone inflation, Region 2 Region 5 physical interactions. The first distinguishable Earth's galactic it would be a Region 4 force was gravity, followed by electromagnetism neighborhood appears collection of different and nuclear interactions. Upon the division of the Strong nuclear Energetic Radiation fairly uniform. regions, each with its universe's forces, matter was created. Galaxy 5 The burning ball that gave rise to the universe remained a Everywhere you look, own particular types source of permanent radiation. Subatomic particles and the types of galaxies of galaxies and each and the background clearly antiparticles annihilated each other. The ball's high density SUPERFORCE spontaneously produced matter and destroyed it. Had this state temperature are distinguishable from Weak nuclear of affairs continued, the universe would never have undergone the essentially the same. the others. EXPANSION growth that scientists believe followed cosmic inflation. Electromagnetism

-43 -38 -12 -4 TIME 0 10 sec 10 sec 10 sec 10 sec 5 sec 3 min

TEMPERATURE - 1032 ° F (and C) 1029 ° F (and C) 1015 ° F (and C) 1012 ° F (and C) 9x109 ° F (5x109 ° C) 2x109 ° F (1x109 ° C)

Scientists theorize that, from At the closest moment to The universe is unstable. Only The universe experiences a Protons and neutrons The electrons and their The nuclei of the nothing, something infinitely zero time, which physics has 10-38 seconds after the big gigantic cooldown. Gravity appear, formed by three antiparticles, lightest elements, small, dense, and hot appeared. 2 been able to reach, the 3 bang, the universe increases in has already become quarks apiece. Because positrons, annihilate hydrogen and 1 temperature is extremely size more than a trillion trillion 4 5 6 7 All that exists today was distinguishable, and the all light is trapped within each other until the helium, form. compressed into a ball smaller than high. Before the universe's inflation, trillion times. The expansion of the electromagnetic force and the strong the web of particles, the universe positrons disappear. The Protons and neutrons unite to the nucleus of an atom. a superforce governed everything. universe and the division of its forces begin. and weak nuclear interactions appear. is still dark. remaining electrons form atoms. form the nuclei of atoms.

ELEMENTARY PARTICLES The neutrinos separate from the initial particle soup through the disintegration In its beginnings, the universe was a soup of particles that interacted with each other 1 sec of neutrons. Though having extremely little mass, the neutrinos might because of high levels of radiation. Later, as the universe expanded, quarks formed the nevertheless form the greatest part of the universe's dark matter. nuclei of the elements and then joined with electrons to form atoms.

Proton FROM PARTICLES TO MATTER The quarks, among the oldest particles, Quark Photon Gluon interact with each other by forces Massless elemental Responsible for transmitted through gluons. Later protons Neutron luminous particle the interactions and neutrons will join to form nuclei. Gluon between quarks

Electron Graviton Quark A gluon interacts Quarks join by means Protons and Negatively charged It is believed to Light, elemental 1 with a quark. 2 of gluons to form 3 neutrons unite to elemental particle transmit gravitation. particle protons and neutrons. create nuclei. 12 WHAT IS THE UNIVERSE? UNIVERSE 13

The Transparent Universe With the creation of atoms and overall cooling, the once opaque and Filament networks THE UNIVERSE TODAY 3 The universe has dense universe became transparent. Electrons were attracted by the large-scale filaments protons of hydrogen and helium nuclei, and together they formed atoms. First filaments that contain millions Star 2 Because of the and millions of galaxies. cluster Photons (massless particles of light) could now pass freely through the gravitational pull of dark universe. With the cooling, radiation remained abundant but was no longer the Gaseous cloud 1 The first gases matter, the gases joined sole governing factor of the universe. Matter, through gravitational force, could in the form of filaments. Star and dust resulting Nebula now direct its own destiny. The gaseous lumps that were present in this from the Big Bang process grew larger and larger. After 100 million years, they formed even form a cloud. larger objects. Their shapes not yet defined, they constituted protogalaxies. Gravitation gave shape to the first galaxies some 500 million years after the big bang, and the first stars began to shine in the densest regions of these Irregular galaxy galaxies. One mystery that could not be solved was why galaxies were Quasar distributed and shaped the way they were. The solution that astronomers have been able to find through indirect evidence is that there exists material called Spiral dark matter whose presence would have played a role in galaxy formation. galaxy

Barred spiral galaxy

DARK MATTER Elliptical The visible objects in the galaxy cosmos represent only a small fraction of the total matter within the universe. Most of it is invisible even to EVOLUTION OF MATTER the most powerful What can be observed in the universe today is a great telescopes. Galaxies and their quantity of matter grouped into galaxies. But that was not stars move as they do the original form of the universe. What the big bang initially 9.1 billion Galaxy cluster because of the gravitational produced was a cloud of uniformly dispersed gas. Just three THE EARTH IS CREATED forces exerted by this million years later, the gas began to organize itself into Like the rest of the planets, the Earth is made of material, which astronomers filaments. Today the universe can be seen as a network of material that remained after the formation of the solar call dark matter. galactic filaments with enormous voids between them. system. The Earth is the only planet known to have life.

TIME (in years) 380,000 500 million 9 billion 13.7 billion

TEMPERATURE 4,900° F (2,700° C) -405° F (-243° C) -432° F (-258° C) -454° F (-270° C)

380,000 years after the big Galaxies acquire their definitive Nine billion years after the big The universe continues to expand. Countless galaxies bang, atoms form. Electrons shape: islands of millions and bang, the solar system are surrounded by dark matter, which represents 22 orbit the nuclei, attracted by millions of stars and masses of emerged. A mass of gas and percent of the mass and energy in the universe. The 8 the protons. The universe 9 10 11 gases and dust. The stars explode dust collapsed until it gave rise ordinary matter, of which stars and planets are becomes transparent. Photons travel as supernovas and disperse heavier to the Sun. Later the planetary system was made, represents just 4 percent of the total. The predominant through space. elements, such as carbon. formed from the leftover material. form of energy is also of an unknown type. Called dark energy, it constitutes 74 percent of the total mass and energy.

FIRST ATOMS NUCLEUS 1 Proton Hydrogen and helium were the first elements to be formed at the atomic level. They are the main components of stars and planets. They are by far the most abundant elements in the universe. TIMESCALE The vast span of time related to the history of appearance of humans on the Earth, and the 11:56 P.M. on December 31, and Columbus sets the universe can be readily understood if it is voyage of Columbus to America. On January 1 sail on the last second of the last day of the Electron scaled to correspond to a single year—a year of this imaginary year—at midnight—the big year. One second on this timescale is equivalent that spans the beginning of the universe, the bang takes place. Homo sapiens appears at to 500 true years. Neutron

BIG BANG THE SOLAR COLUMBUS'S occurs on the SYSTEM ARRIVAL Helium Carbon first second of is created on takes place on 2 Since the nucleus 3 With time, heavier and more complex elements Hydrogen the first day of August 24 of the last second has two protons, were formed. Carbon, the key to human life, has six 1 An electron is attracted by the year. this timescale. of December 31. and orbits the nucleus, which two electrons are protons in its nucleus and six electrons orbiting it. has a proton and a neutron. NUCLEUS 2 attracted to it. JANUARY DECEMBER 14 WHAT IS THE UNIVERSE? UNIVERSE 15

Everything Comes to an End DISCOVERIES The key discovery that led to the big bang theory was made in the early he big bang theory helped solve the enigma of the early moments of the universe. What has yet to 1920s by Edwin Hubble, who be resolved is the mystery surrounding the future that awaits. To unravel this mystery, the total discovered that galaxies were moving T away from each other. In the 1940s, mass of the universe must be known, but that figure has not yet been reliably determined. The George Gamow developed the idea most recent observations have removed some of this uncertainty. It seems that the mass of the universe that the universe began with a primordial explosion. A consequence 1965 is far too little to stop its expansion. If this is this case, the universe's present growth is merely the last of such an event would be the 1920s 1940s GALACTIC EXPANSION GAMOW'S SUSPICION BACKGROUND RADIATION step before its total death in complete darkness. existence of background radiation, By noting a redshift toward the red end of the Gamow first hypothesized the big bang, Penzias and Wilson detected radio signals which Arno Penzias and Robert spectrum, Hubble was able to demonstrate that holding that the early universe was a that came from across the entire sky—the Wilson accidentally detected in the galaxies were moving away from each other. “cauldron” of particles. uniform signal of background radiation. mid-1960s.

The universe The universe's Gravity is not Flat Universe 1 continuously 2 expansion is 3 sufficient to bring a Self-generated expands and unceasing but complete stop to the There is a critical amount of mass evolves. ever slower. universe's expansion. Universes for which the universe would 1 expand at a declining rate without A less widely accepted theory about ever totally stopping. The result of this 3 the nature of the universe suggests eternal expansion would be the existence of that universes generate themselves. an ever-increasing number of galaxies and If this is the case, universes would be created continuously like the branches of a stars. If the universe were flat, we could Universe 1 The universe Universe 4 tree, and they might be linked by talk about a cosmos born from an explosion, 4 expands indefinitely. but it would be a universe continuing Universe 3 supermassive black holes. Black outward forever. It is difficult to think hole about a universe with these characteristics.

Black hole THE HAWKING UNIVERSE The universe was composed originally of four Open Universe BLACK HOLES spatial dimensions without the dimension of Some theorists believe time. Since there is no change without time, After the original Expansion is reaches a point where The most accepted theory about that, by entering a 1 expansion, the 2 continuous and 3 everything grows dark one of these dimensions, according to Hawking, the future of the cosmos says black hole, travel universe grows. pronounced. and life is extinguished. 4 transformed spontaneously on a small scale that the universe possesses a through space to Inflection point into a temporal dimension, and the universe BIG BANG TIME mass smaller than the critical value. The other universes might began to expand. latest measurements seem to indicate that be possible because of the present time is just a phase before the antigravitational death of the universe, in which it goes effects. completely dark.

Object in three Object that changes New universe dimensions with time

Universe 1 Universe 2 Universe 3

HOW IT IS MADE UP Closed Universe Dark energy is hypothesized to be the predominant energy in the If the universe had more than universe. It is believed to speed up critical mass, it would expand BIG CRUNCH the expansion of the universe. 2 until reaching a point where gravity stopped the expansion. Then, the universe would contract in the Big 74% Crunch, a total collapse culminating in dark energy an infinitely small, dense, and hot spot Baby Universes similar to the one from which the 22% According to this theory, universes Big Crunch, would give rise to a supermassive universe was formed. Gravity's pull on dark matter continuously sprout other universes. But black hole, from which another universe would the universe's excess matter would stop The universe The universe's The universe collapses 5 in this case, one universe would be be born. This process could repeat itself the expansion and reverse the process. 1 expands violently. 2 growth slows. 3 upon itself, forming a dense, hot spot. 4% created from the death or disappearance of indefinitely, making the number of universes visible matter another. Each dead universe in a final collapse, or impossible to determine. 16 WHAT IS THE UNIVERSE? UNIVERSE 17

UNIVERSAL GRAVITATION NEWTON'S EQUATION The Forces of the Universe Two bodies with mass attract each other. Whichever The gravitation proposed by Newton is Newton's law, an accepted paradigm the mutual attraction between bodies until Einstein's theory of general body has the greatest mass will exert a greater force on the other. The greater the distance between the he four fundamental forces of nature are those that are not derived from basic forces. Physicists having mass. The equation developed by relativity, lies in its failure to make time objects, the smaller the force they exert on each other. Newton to calculate this force states an essential component in the believe that, at one time, all physical forces functioned as a single force and that during the F that the attraction experienced by two interaction between objects. According m1 m2 T expansion of the universe, they became distinct from each other. Each force now governs different bodies is directly proportional to the to Newton, the gravitational attraction product of their masses and inversely between two objects with mass did not processes, and each interaction affects different types of particles. Gravity, electromagnetism, strong proportional to the square of the depend on the properties of space but d nuclear interactions, and weak nuclear interactions are essential to our understanding of the behavior of distance between them. Newton was an intrinsic property of the objects represented the constant of themselves. Nevertheless, Newton's law x x the many objects that exist in the universe. In recent years, many scientists have tried with little success proportionality resulting from this of universal gravitation was a F=G m1 m2 to show how all forces are manifestations of a single type of exchange. interaction as G. The shortcoming of foundation for Einstein's theory. d2

What Real Strong Nuclear Force General Theory of Relativity we see position The strong nuclear force holds the protons and neutrons The biggest contribution to our comprehension of the universe's internal of atomic nuclei together. Both protons and neutrons are workings was made by Albert Einstein in 1915. Building on Newton's 3 subject to this force. Gluons are particles that carry the theory of universal gravitation, Einstein thought of space as linked to time. To strong nuclear force, and they bind quarks together to form LUMINOUS T Newton, gravity was merely the force that attracted two objects, but Einstein protons and neutrons. Atomic nuclei are held together by hypothesized that it was a consequence of what he called the curvature of space- residual forces in the interaction between quarks and gluons. time. According to his general theory of relativity, the universe curves in the RA presence of objects with mass. Gravity, according to this theory, is a distortion of JEC

TORY space that determines whether one object rolls toward another. Einstein's general Quarks and gluons theory of relativity required scientists to consider the universe in terms of a non- Positive Electromagnetism 1 The strong nuclear interaction Euclidian geometry, since it is not compatible with the idea of a flat universe. pole takes place when the gluon Nucleus In Einsteinian space, two parallel lines can meet. Electromagnetism is the force that affects interacts with quarks. electrically charged bodies. It is involved in the 2 chemical and physical transformations of the atoms and molecules of the various elements. The electromagnetic force can be one of attraction or repulsion, with two types of charges or poles. Quark

Union Force 2 Quarks join and form Gluon nuclear protons and Attraction 2 Hydrogen neutrons. SUN Two atoms are drawn together, E=mc and the electrons rotate around the new molecule. In Einstein's equation, energy and mass are Helium interchangeable. If an object increases its mass, its energy increases, and vice versa. EARTH Force

Positive Weak Nuclear Force Electron pole Negative The weak nuclear force is not as strong as the other Gravity pole forces. The weak nuclear interaction influences the beta Gravity was the first force to mass in space would deform the cube. 4 decay of a neutron, which releases a proton and a become distinguishable from the Gravity can act at great distances (just as Nucleus neutrino that later transforms into an electron. This force takes 1 original superforce. Today electromagnetism can) and always exerts a part in the natural radioactive phenomena associated with certain scientists understand gravity in Einstein's force of attraction. Despite the many types of atoms. terms as an effect of the curvature of attempts to find antigravity (which could Negative space-time. If the universe were thought of counteract the effects of black holes), it pole Hydrogen HELIUM ISOTOPE as a cube, the presence of any object with has yet to be found. 1 A hydrogen atom interacts with a weak, light particle MOLECULAR MAGNETISM (WIMP). A neutron's bottom quark transforms Electron In atoms and molecules, the electromagnetic force is into a top quark. dominant. It is the force that causes the attraction between protons and electrons in an atom and the Proton attraction or repulsion between ionized atoms. HYDROGEN ATOM

Proton Helium BENDING LIGHT 2 The neutron transforms Light also bends because of the curvature of space-time. Electron into a proton. An electron When seen from a telescope, the real position of an object is released, and the helium isotope that is formed has is distorted. What is perceived through the telescope is a no nuclear neutrons. The universe, if it were empty, could be The universe is deformed by the mass false location, generated by the curvature of the light. It pictured in this way. of the objects it contains. is not possible to see the actual position of the object. Neutron WIMP ETA CARINAE NEBULA LUMINOUS 20-21 THE FINAL DARKNESS 30-31 With a diameter of more than 200 light-years, it is one of biggest and brightest nebulae of our galaxy. STELLAR EVOLUTION 22-23 ANATOMY OF GALAXIES 32-33 This young, supermassive star is expected to become What Is in the Universe? a supernova in the near future. RED, DANGER, AND DEATH 24-25 ACTIVE GALAXIES 34-35 GAS SHELLS 26-27 STELLAR METROPOLIS 36-37 SUPERNOVAE 28-29

he universe is populated on a other, triggering the formation of stars. that make up, for example, the Eta without a doubt that most of the atoms grand scale by strands of In the vast cosmos, there are also Carinae Nebula (shown), which is that make up our bodies have been born superclusters surrounding quasars, pulsars, and black holes. composed of jets of hot, fluorescent in the interior of stars. T vacant areas. Sometimes the Thanks to current technology, we can gases. Although not all the objects in the galaxies collide with each enjoy the displays of light and shadow universe are known, it can be said 20 WHAT IS IN THE UNIVERSE? UNIVERSE 21 SCORPIUS REGION

Measuring Distance Luminous When the Earth orbits the Sun, the closest stars appear to move in front of a background of more or a long time stars were a mystery to humans, and it was only as recently distant stars. The angle described by the movement of a star in a six-month period of the Earth's rotation is called as the 19th century that astronomers began to understand the true nature its parallax. The parallax of the most distant stars are too F of stars. Today we know that they are gigantic spheres of incandescent small to measure. The closer a star is to the Earth, the greater its parallax. gas—mostly hydrogen, with a smaller proportion of helium. As a star radiates GLOBULAR CLUSTER light, astronomers can precisely measure its brightness, color, and temperature. More than a million stars are The parallax of star B grouped together into a spherical PARALLAX is greater than that of Because the parallax Because of their enormous distance from the Earth, stars beyond the Sun only cluster called Omega Centauri. star A, so we see that of star A is small, we B is closer to the see that it is distant Earth. appear as points of light, and even the most powerful telescopes do not reveal from the Earth. any surface features. A

Hertzsprung-Russell (H-R) Diagram B The H-R diagram plots the intrinsic those with greatest intrinsic luminosity. luminosity of stars against their They include blue stars, red giants, and

spectral class, which corresponds to their red supergiants. Stars spend 90 percent OPEN CLUSTER Position of Position of temperature or the wavelengths of light of their lives in what is known as the The Pleiades are a formation of the Earth in SUN the Earth in they emit. The most massive stars are main sequence. some 400 stars that will eventually January July move apart.

INTRINSIC LUMINOSITY (SUN = 1) TYPE O 52,000-72,000° F (29,000-40,000° C) 100,000 Supergiants Red giants TYPE B 17,500-52,000° F 10,000 Spectral Analysis (9,700-29,000° C) The electromagnetic waves that make up light have 1,000 TYPE A different wavelengths. When light from a hot 13,000-17,500° F 100 (7,200-9,700° C) object, such as a star, is split into its different wavelengths, a band of colors, or spectrum, is obtained. 10 TYPE F Patterns of dark lines typically appear in the spectrum of 10,500-13,000° F a star. These patterns can be studied to determine the 1 Main (5,800-7,200° C) sequence SUN elements that make up the star. 0.1 TYPE G 8,500-10,500° F Calcium Hydrogen Hydrogen Sodium Hydrogen 0.01 (4,700-5,800° C)

0.001 White dwarfs TYPE K 6,000-8,500° F (3,300-4,700° C) 0.0001 Wavelength longest on the red side OB A F G KM TYPE M 4,000-6,000° F (2,100-3,300° C) DOPPLER EFFECT SPECTRAL CLASSES When a star moves toward or away from an observer, its wavelengths of light shift, a phenomenon called the Doppler effect. If the star is approaching the Earth, the dark lines in its spectrum experience a blueshift. If it moves away from the Earth, the lines Light-years and Parsecs experience a redshift. In measuring the great distances year is a unit of distance, not time. A parsec between stars, both light-years (ly) is equivalent to the distance between the COLORS The hottest stars are Wavelength is compressed by the movement of the star. and parsecs (pc) are used. A light-year is star and the Earth if the parallax angle is of bluish-white (spectral classes the distance that light travels in a year— one second arc. A pc is equal to 3.26 light- O, B, and A). The coolest stars Star Earth 5.9 trillion miles (10 trillion km). A light- years, or 19 trillion miles (31 trillion km). are orange, yellow, and red (spectral classes G, K, and M).

PRINCIPAL STARS WITHIN 100 LY FROM THE SUN Dark lines deviate toward the blue end of the spectrum. BLUESHIFT of a star moving toward the Earth. SUN ALPHA SIRIUS PROCYON ALTAIR VEGA POLLUX ARCTURUS CAPELLA CASTOR ALDEBARAN ALIOTH (G2) CENTAURI (A0 and (F5 and (A7) (A0) (K0 giant) (K2 giant) (G6 and G2 (A2, A1, and M1) (K5 giant) (A0 giant) (G2, K1, M5) dwarf star) dwarf star) giants) REGULUS MENKALINAN GACRUX ALGOL (B7 and K1) (A2 and A2) (M4 giant) (B8 and K0)

LIGHT-YEARS

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 PARSECS 22 WHAT IS IN THE UNIVERSE? UNIVERSE 23

SUPERNOVA When the star can no longer fuse any more elements, its core collapses, causing a strong emission of energy. Stellar Evolution RED SUPERGIANT 4. The star swells and heats up. tars are born in nebulae, which are giant clouds of gas (mainly hydrogen) 3. Through nuclear reactions, a and dust that float in space. Stars can have a life span of millions, heavy core of iron is formed. S or even billions, of years. The biggest stars have the shortest lives, because they consume their nuclear fuel (hydrogen) at a very accelerated rate. Other stars, like the Sun, burn fuel at a slower rate and may live some 10 billion years. Many times, a star's size indicates its age. Smaller stars are the youngest, and bigger stars are approaching STAR their end, either through cooling or by A star is finally born. It 2. fuses hydrogen to form exploding as a supernova. helium and lies along the main sequence.

PROTOSTAR A protostar has a 1. dense, gaseous core surrounded by Massive star a cloud of dust. More than 8 solar masses

BLACK HOLE If the star's initial mass is 20 solar masses or more, its 5. nucleus is denser and it turns into a Nebula black hole, whose gravitational force is extremely strong. NEUTRON STAR A CLOUD OF GAS AND DUST collapses If the star's initial mass is between because of gravitational forces. In doing 5. eight and 20 solar masses, it ends up so it heats up and divides into smaller as a neutron star. clouds. Each one of these clouds will form a protostar.

BLACK DWARF If a white dwarf 6. fades out completely, it becomes a black Small star dwarf. Less than 8 solar masses

RED GIANT The star continues to expand, but its mass remains 3. constant and its core heats up. WHITE DWARF Life Cycle of a Star When the star's helium is depleted, The star remains it fuses carbon and oxygen. 5. surrounded by The evolution of a star depends on its mass. The gases and is dim. smallest ones, like the Sun, have relatively long and PROTOSTAR modest lives. Such a star begins to burn helium when its A protostar is formed hydrogen is depleted. In this way, its external layers 1. by the separation of gas begin to swell until the star turns into a red giant. It and dust. Gravitational ends its life as white dwarfs, eventually fading away effects cause its core to completely, ejecting remaining outer layers, and forming rotate. a planetary nebula. A massive star, because of its higher density, can form elements heavier than helium from its nuclear reactions. In the final stage of its life, its core 95% of stars collapses and the star explodes. All that remains is a STAR The star shines and PLANETARY NEBULA When layers detach, expelling end their lives as white dwarfs. Other (larger) hyperdense remnant, a neutron star. The most massive slowly consumes its the star's fuel is depleted, its gases in an expanding shell stars explode as supernovae, illuminating stars end by forming black holes. 2. hydrogen. It begins to fuse 4. core condenses, and its outer of gases. galaxies for weeks, although their brightness is helium as its size increases. often obscured by the gases and dust. 24 WHAT IS IN THE UNIVERSE? UNIVERSE 25

SUN 1% White Dwarf Red, Danger, and Death After going through the red-giant stage, a solar-type star loses its The scale of the 3 4 diameter of the Sun outer layers, giving rise to a planetary nebula. In its center remains a hen a star exhausts its hydrogen, it begins to die. The to the diameter of white dwarf—a relatively small, very hot (360,000° F [200,000° C]), dense a typical red giant. star. After cooling for millions of years, it shuts down completely and helium that now makes up the star's core begins to 2 becomes a black dwarf. W undergo nuclear reactions, and the star remains 5 bright. When the star's helium is depleted, fusion of 6 carbon and oxygen begins, which causes the star's 1 LIFE CYCLE OF A NEBULaNGC 6751 core to contract. The star continues to live, though its STAR 7 surface layers begin to expand and cool as the star turns into a red giant. Stars similar to the Sun (solar-type stars) 7 follow this process. After billions of years, they end up as 2 6 white dwarfs. When they are fully extinguished, they will 3 5 be black dwarfs, invisible in space. 4 Red giant

Red Giant

All stars go through a red-giant from a lack of hydrogen. A supergiant stage. Depending on a star's star (one with an initial mass greater mass, it may collapse or it may simply than eight solar masses) lives a much Hot Spots die enveloped in gaseous layers. The shorter life. Because of the high density Hot spots appear when large core of a red giant is 10 times smaller attained by its core, it eventually jets of incandescent gas reach the star's surface. than it was originally since it shrinks collapses in on itself and explodes. They can be detected on the WHITE DWARF surface of red giants. After the nuclear reaction in the star's core ceases, the star ejects its outer layers, which then form a planetary nebula. DIAMETER Sun Convection

Red supergiant. Placed Mercury's orbit Cells at the center of the Convection cells carry heat toward solar system, it would Venus's orbit the surface of a star. The ascending HERTZSPRUNG-RUSSELL swallow up Mars and currents of gas eventually reach When a white dwarf leaves the Jupiter. Earth's orbit the surface of the star, carrying red-giant stage, it occupies the with them a few elements that Mars's orbit lower-left corner of the H-R formed in the star's core. Dust Grains Dust grains condense in the star's outer diagram. Its temperature may be atmosphere and later disperse in the form of double that of a typical red giant. stellar winds. The dust acquires a dark A massive white dwarf can appearance and is swept into interstellar collapse in on itself and end its Red giant. Placed at space, where new generations of stars will the center of the solar Jupiter's orbit form. The outer layer of the star may life as a neutron star. system, it could reach extend across several light-years of only the nearer planets, Saturn's orbit interstellar space. such as Mercury, Venus, and the Earth. REGION OF THE CORE HYDROGEN SPECTACULAR DIMENSIONS 1 Hydrogen continues undergoing On leaving the main sequence, size decreases to between 10 and nuclear fusion in the exterior of the core even when the inner Mars Venus Mars Venus Mars Venus the star enlarges to 200 times 100 times the size of the Sun. core has run out of hydrogen. Sun Sun the size of the Sun. When the The star then remains stable until star begins to burn helium, its it becomes a white dwarf. HELIUM 2 Helium is produced by the fusion Sun of hydrogen during the main HERTZSPRUNG-RUSSELL sequence. When the star exhausts its hydrogen, it leaves the main 3 CARBON AND OXYGEN sequence and burns helium Carbon and oxygen are produced by the fusion of helium within the Earth Mercury Earth Mercury Earth as a red giant (or a core of the red giant. supergiant). The smallest Earth stars take billions of years to leave the main sequences. TEMPERATURE THE FUTURE OF THE SUN 4 As the helium undergoes fusion, The color of a red giant is Like any typical star, the Sun burns hydrogen brightness and expanding until it swallows the temperature of the core during its main sequence. After taking Mercury. At its maximum size, it may even RED GIANT caused by its relatively cool reaches millions of degrees approximately five billion years to exhaust its envelop the Earth. Once it has stabilized, it will The radius of the surface temperature of Fahrenheit (millions of degrees supply of hydrogen, it will begin its continue as a red giant for two billion years and Sun reaches the 3,600° F (2,000° C). Celsius). transformation into a red giant, doubling in then become a white dwarf. Earth's orbit. 26 WHAT IS IN THE UNIVERSE? UNIVERSE 27

NGC 6542 CAT'S EYE HELIX Gas Shells 3 4 hen a small star dies, all that remains is an expanding 2 gas shell known as a planetary nebula, which has 5 W nothing to do with the planets. In general, 6 planetary nebulae are symmetrical or spherical LIFE CYCLE OF 1 objects. Although it has not been possible to A STAR 7 determine why they exist in such diversity, the reason may be related to the effects of the magnetic field of the 7 6 3 tons dying central star. Viewed through a telescope, several 2 is the weight of a single nebulae can be seen to contain a central dwarf star, a mere 3 5 tablespoon of a white 4 Concentric remnant of its precursor star. dwarf. A white dwarf is very massive in spite of Planetary circles nebula of gas, resembling the inside of an onion, form a the fact that its multilayered structure around the white dwarf. diameter of 9,300 miles M2-9 Each layer has a mass greater than the combined (or 15,000 km) is The Butterfly Nebula contains mass of all the planets in the solar system. BUTTERFLY a star in addition to a white comparable to the dwarf. Each orbits the other Earth's. inside a gas disk that is 10 times larger than Pluto's orbit. The Butterfly Nebula NGC 7293 is located 2,100 light-years The Helix is a from Earth. planetary nebula that was created at the end of the life of a HOURGLASS solar-type star. It is 650 light-years from the Earth and is located in the TWICE THE constellation Aquarius. TEMPERATURE OF THE SUN is reached at the surface of a White white dwarf, causing it to appear white even though its Dwarf luminosity is a thousand times less than that of the Sun. The remains of the red giant, in which the fusion of carbon and oxygen has ceased, lie at the center of the SPIROGRAPH nebula. The star slowly cools and fades. MYCN 18 IC 418 The two rings of colored The Spirograph Nebula has a gas form the silhouette of hot, luminous core that this hourglass-shaped excites nearby atoms, nebula. The red in the causing them to glow. The photograph corresponds to Spirograph Nebula is about nitrogen, and the green 0.1 light-year wide and is corresponds to hydrogen. located 2,000 light-years This nebula is 8,000 light- from Earth. years from the Earth.

LARGER DIAMETER CHANDRASEKHAR Hydrogen Less massive white dwarf LIMIT The continuously expanding masses of gas surrounding the The astrophysicist star contain mostly hydrogen, Subrahmanyan with helium and lesser amounts Chandrasekhar, winner of the of oxygen, nitrogen, and other Nobel Prize for Physics in elements. 1983, calculated the maximum mass a star could have so that SMALLER DIAMETER it would not eventually collapse More massive white dwarf on itself. If a star's mass exceeds this limit, the star will eventually explode in a supernova.

DENSITY OF A WHITE DWARF 1.44 SOLAR MASSES The density of a white dwarf is a million The mass of a star is indirectly is the limit Chandrasekhar times greater than the density of water. proportional to its diameter. A white obtained. In excess of this value, In other words, each cubic meter of a dwarf with a diameter 100 times smaller a dwarf star cannot support its white dwarf star weighs a million tons. than the Sun has a mass 70 times greater. own gravity and collapses. 28 WHAT IS IN THE UNIVERSE?

Explosion 3 4 Supernova Supernovae Other Elements The star's life ends in an immense When a star's iron core upon itself. The resulting explosion. During the weeks supernova is an extraordinary explosion of a giant star at 2 increases in density to 1.44 explosion causes the following the explosion, great 5 solar masses, the star can formation of elements that quantities of energy are radiated the end of its life, accompanied by a sudden increase no longer support its own are heavier than iron, such that are sometimes greater than the weight and it collapses as gold and uranium. energy emitted by the star's parent A in brightness and the release of a great amount 6 galaxy. A supernova may illuminate its galaxy for weeks. of energy. In 10 seconds, a supernova releases 100 1 LIFE CYCLE OF times more energy than the Sun will release in its A STAR 7 entire life. After the explosion of the star that gives 7 rise to a supernova, the gaseous remnant expands and 6 shines for millions of years. It is estimated that, in our Milky 2 3 Way galaxy, two supernovae occur per century. 4 5

FEBRUARY 22, The Twilight of a Star 1987 This star is in its last The explosion that marks the its remaining gases, which will expand moments of life. Because it end of a supergiant's life occurs and shine for hundreds—or even is very massive, it will end because the star's extremely heavy thousands—of years. The explosion of its life in an explosion. The core has become incapable of the star injects new material into galaxy exhibits only its usual THE END SUPERMASSIVE ETA CARINAE Either a neutron star luminosity. supporting its own gravity any longer. interstellar space and contributes The mass of Eta Carinae In the absence of fusion in its interior, heavy atoms that can give rise to new or a black hole may is 100 times greater the star falls in upon itself, expelling generations of stars. form depending on the than that of the Sun. initial mass of the star Astronomers believe that has died. that Eta Carinae is about to explode, but no one knows Core when. A star's core can be seen to be separated into distinct layers that correspond to the different elements created during nuclear fusion. The last element created before the star's FEBRUARY collapse is iron. 23, 1987 After the supernova CRAB NEBULA explosion, increased brightness is observed in the GAS AND DUST Gas and dust that have region near the star. accumulated in the two visible lobes absorb the blue light and ultraviolet FUSION The nuclear rays emitted from its center. reactions in a dying star occur BEFORE AND AFTER at a faster rate The image at left shows a sector of years from the Earth, depicted before than they do in a the Large Magellanic Cloud, an the explosion of supernova 1987A. The red giant. irregular galaxy located 170,000 light- image at right shows the supernova. Stellar Remnant

When the star explodes as a supernova, it Supergiant leaves as a legacy in space the heavy The diameter of the star may elements (such as carbon, oxygen, and iron) that increase to more than 1,000 times that of the Sun. Through were in the star's nucleus before its collapse. The nuclear fusion, the star can Crab Nebula (M1) was created by a supernova produce elements even heavier seen in 1054 by Chinese astronomers. The Crab than carbon and oxygen. Nebula is located 6.5 light-years from Earth and DENSE has a diameter of six light-years. The star that CORE gave rise to the Crab Nebula may have had an initial mass close to 10 solar masses. In 1969, a GASEOUS FILAMENTS pulsar radiating X-rays and rotating 33 times per Gaseous filaments are ejected by the supernova at 620 miles second was discovered at the center of the (1,000 km) per second. nebula, making the Crab Nebula a very powerful source of radiation. 30 WHAT IS IN THE UNIVERSE? UNIVERSE 31

1 Neutron Star LOSS OF MASS The Final Darkness 3 4 RED GIANT Toward the end of When a star's initial mass is between A red giant leaves its life, a neutron 10 and 20 solar masses, its final mass the main sequence. star loses more than he last stage in the evolution of a star's core is its 2 will be larger than the mass of the Sun. Its diameter is 100 90 percent of its Black hole 5 Despite losing great quantities of matter times greater than initial mass. transformation into a very dense, compact stellar body. during nuclear reactions, the star finishes the Sun's. T with a very dense core. Because of its intense Its particulars depend upon the amount of mass 6 4 magnetic and gravitational fields, a neutron 2 involved in its collapse. The largest stars become black star can end up as a pulsar. A pulsar is a DENSE CORE LIFE CYCLE OF The core's exact holes, their density so great that their gravitational 1 A STAR rapidly spinning neutron star that gives off a SUPERGIANT 7 A supergiant grows composition is forces capture even light. The only way to detect these beam of radio waves or other radiation. As and rapidly fuses presently unknown. Neutron star 7 the beam sweeps around the object, the heavier chemical Most of its dead stars is by searching for the effects of their radiation is observed in very regular pulses. elements, forming interacting particles carbon, oxygen, and are neutrons. gravitation. 2 6 finally iron. 3 5 4 Strong Gravitational 3 Attraction 1 billion EXPLOSION The gravitational force of the black hole attracts The star's iron core tons is what one tablespoon of a Discovery of Black Holes gases from a neighboring star. This gas forms a collapses. Protons neutron star would weigh. Its small large spiral that swirls faster and faster as it gets and electrons diameter causes the star to have a The only way of detecting the presence of X-rays. The black hole, by exerting such closer to the black hole. The gravitation field that annihilate each other compact, dense core accompanied by a black hole in space is by its effect on powerful gravitational force, attracts everything it generates is so strong that it traps objects that and form neutrons. intense gravitational effects. neighboring stars. Since the gravitational force that passes close to it, letting nothing escape. pass close to it. exerted by a black hole is so powerful, the gases Since even light is not exempt from this of nearby stars are absorbed at great speed, phenomenon, black holes are opaque and spiraling toward the black hole and forming a invisible to even the most advanced telescopes. structure called an accretion disk. The friction Some astronomers believe that X-RAYS As gases enter the of the gases heats them until they shine supermassive black holes might have black hole, they are Pulsars brightly. The hottest parts of the accretion disk a mass of millions, or even heated and emit X-rays. CURVED SPACE The first pulsar (a neutron star radiating may reach 100,000,000° C and are a source of billions, of solar masses. The theory of relativity suggests depth of which depends on the radio waves) was discovered in 1967. that gravity is not a force but a mass of the object. Objects are Pulsars rotate approximately 30 times per second distortion of space. This distortion attracted to other objects through and have very intense magnetic fields. Pulsars creates a gravitational well, the the curvature of space. emit radio waves from their two magnetic poles LIGHT RAYS when they rotate. If a pulsar absorbs gas from a Accretion Disk neighboring star, a hot spot that radiates X-rays An accretion disk is a gaseous accumulation is produced on the pulsar's surface. of matter that the black hole draws from Total escape nearby stars. In the regions of the disk Rays of light that very close to the black hole, X-rays are pass far from the emitted. The gas that accumulates center of a black rotates at very high speeds. When hole continue STRUCTURE OF A PULSAR the gases from other stars unaffected. collide with the disk, they Rotation axis create bright, hot spots. 1 THE SUN forms a shallow gravitational well. Magnetic field

A WHITE 2 DWARF Close to the limit generates a Since the rays of light deeper have not crossed the gravitational event horizon, they A NEUTRON STAR ENTRANCE 3 attracts objects at well, drawing still retain their speeds approaching half in objects at a Possible brightness. the speed of light. The higher speed. solid core gravitational well is even more pronounced. Darkness Rays of light that pass close to the EXIT core of a black hole Neutron are trapped. BLACK HOLE star The objects that approach CROSS SECTION 4 the black hole too closely are swallowed by it. The black hole's Radio-wave beam ACCRETION X-RAYS gravitational well is infinite and traps DISK matter and light forever. The event WORMHOLE horizon describes the limit of what is, and is not, absorbed. Any object that crosses the event horizon follows a spiral Devouring gas from path into the gravitational well. Some HOT BLACK scientists believe in the existence of so- a supergiant GASES HOLE Bright gases called wormholes—antigravity tunnels, through which travel across the universe Located within a binary system, the pulsar can Since the accretion disk is fed by gases is hypothesized to be possible. By taking follow the same process as a black hole. The pulsar's spinning at high speed, it shines advantage of the curvature of space, scientists gravitational force causes it to absorb the gas of intensely in the region closest to its core think it could be possible to travel from the smaller, neighboring stars, heating up the pulsar's but at its edges is colder and darker. Earth to the Moon in a matter of seconds. surface and causing it to emit X-rays. 32 WHAT IS IN THE UNIVERSE? UNIVERSE 33

MILKY WAY Seen from its side, the Milky Way looks like a flattened disk, swollen at the center. Around the disk is a spherical region, called a halo, ngc 6205 hercules Anatomy of Galaxies containing dark matter and globular clusters of stars. From June to September, the Milky Way is especially bright, something that alaxies are rotating groups of stars, would make it more visible viewed from above gas, and dust. More than 200 years ago, than from the side. G philosopher Immanuel Kant postulated that nebulae were island-universes of distant CLASSIFYING GALAXIES ACCORDING TO HUBBLE stars. Even though astronomers now know that galaxies are held together by gravitational force, they have not been able to decipher what reasons might be behind galaxies' many shapes. The various types of galaxies range from ovals of old stars to spirals with arms of young stars and bright gases. The center of a galaxy has the greatest accumulation of stars. The Milky Way ELLIPTICAL SPIRAL IRREGULAR Galaxy is now known to be so big that rays of light, which travel at These galaxies are elliptical in In a spiral galaxy, a nucleus of Irregular galaxies have no defined 186,000 miles (300,000 km) per second, take 100,000 years to cross shape and have little dust and old stars is surrounded by a flat shape and cannot be classified. gas. Their masses fall within a disk of stars and two or more They contain a large amount of from one end to the other. wide range. spiral arms. gases and dust clouds. Galactic Clusters SUBCLASSIFICATIONS Galaxies are objects that tend to form groups or clusters. Galaxies are subdivided into and barred spiral galaxies). An E0 Acting in response to gravitational force, they can form different categories according to galaxy is elliptical but almost clusters of galaxies of anywhere from two to thousands of their tendency toward round circular, and an E7 galaxy is a galaxies. These clusters have various shapes and are thought to shape (in the case of elliptical flattened oval. An Sa galaxy has a galaxies), as well as by the large central axis and coiled arms, expand when they join together. The Hercules cluster, shown here, presence of an axis and the length and an Sc galaxy has a thinner axis was discovered by Edmond Halley in 1714 and is located of their arms (in the case of spiral and more extended arms. approximately 25,100 light-years from Earth. Each dot represents a galaxy that includes billions of stars.

Star Cities The first galaxies formed 100 million years after the big bang. Billions of these great conglomerates of stars can be found throughout space. The two most important discoveries night sky were actually distant galaxies. Hubble's concerning galaxies are attributed to the discovery put an end to the view held by astronomers astronomer Edwin Hubble. In 1926, he pointed out at the time that the Milky Way constituted the of the spectrum of light radiated by the stars in the that the spots, or patches, of light visible in the universe. In 1929, as a result of various observations galaxies, Hubble noted that the light from the galaxies showed a redshift (Doppler effect). This effect indicated that the galaxies were moving away from the Milky Way Galaxy. Hubble concluded that the universe is expanding. But the expansion of the merge. When two galaxies collide, they can distort shapes. The Sombrero Galaxy, shown in the center of universe does not imply that galaxies are growing in each other in various ways. Over time, there are fewer the page, has a bright white core surrounded by thin COLLISION numbers. On the contrary, galaxies can collide and and fewer galaxies. Some galaxies exhibit very peculiar spiral arms. 300 million light-years from the Earth, these ngc 4676 two colliding galaxies form a pair. Together they are called “The Mice” for the large tail 1 2 3 4 5 of stars emanating from each galaxy. With 1.2 BILLION 300 MILLION 300 MILLION 300 MILLION NOW time, these galaxies will YEARS YEARS YEARS YEARS two jets of fuse into a single, larger ago, the Antennae later, the go by until the later, the expelled one. It is believed that (NGC 4038 and galaxies collision takes stars in the stars stretch in the future the NGC 4039) were collided at place and the spiral arms far from the universe will consist of two separate great speed. shapes of the are expelled original a few giant stars. spiral galaxies. galaxies are from both galaxies. distorted. galaxies. 34 WHAT IS IN THE UNIVERSE? UNIVERSE 35

CLASSIFICATION Galaxy Formation The classification of an active galaxy depends Active Galaxies upon its distance from Earth and the perspective A theory of galaxy formation fury and became inactive. According from which it is seen. Quasars, radio galaxies, and associated with active galaxies to this theory, there is a natural blazars are members of the same family of objects holds that many galaxies, possibly progression from quasars to active small number of galaxies differ from the rest by emitting high amounts of and differ only in the way they are perceived. including the Milky Way, were formed galaxies to the common galaxies of energy. The energy emission might be caused by the presence of black holes in from the gradual calming of a quasar today. In 1994, astronomers studying A its core that were formed through the gravitational collapse accompanying the at their core. As the surrounding the center of the Milky Way QUASARS The most gases consolidated in the formation of discovered a region that may contain death of supermassive stars. During their first billion years, the galaxies might have powerful objects in the stars, the quasars, having no more a black hole and could be left over universe, quasars are so distant gases to absorb, lost their energetic from early galactic activity. accumulated surrounding gaseous disks with their corresponding emissions of radiation. It from Earth that they appear to us as diffuse stars. They are the is possible that the cores of the first galaxies are the quasars that are now observed at very bright cores of remote galaxies. great distances. GASEOUS CLOUDS Gaseous clouds appeared from the gravitational collapse of immense masses Radio galaxies are RADIO GALAXIES of gas during the early stages of the the largest objects in the universe. Jets of universe. Later, in the clouds' interior, GAS Energetic Activity gases come out from their centers that stars began to form. As two jets are expelled from extend thousands of light-years. The cores the core, radio waves are Astronomers believe that active galaxies are born from a supermassive black hole with a of radio galaxies cannot be seen. emitted. If the waves collide with clouds of a direct legacy from the beginning of the quasar that became inactive as stars formed and intergalactic gas, they universe. After the big bang, these galaxies would it was left without gas to feed it. This process swell and form have retained very energetic levels of radiation. of formation might be common to many gigantic clouds that Quasars, the brightest and most ancient objects in galaxies. Today quasars represent the INCREASING can emit radio the universe, make up the core of this type of limit of what it is possible to see, waves or X-rays. BLAZARS Blazars may be GRAVITY galaxy. In some cases, they emit X-rays or radio even with specialized active galaxies with jets of gas Dark clouds of gas waves. The existence of this high-energy activity telescopes. Quasars are that are aimed directly toward 1 and dust on the helps support the theory that galaxies could be small, dense, and bright. Earth. The brightness of a blazar As the gases outer edge of a varies from day to day. 2 move inward, black hole are their temperature gradually The strong increases. swallowed up. 3 gravitational force of the disk attracts and destroys stars.

The center of the 4 black hole radiates charged particles.

CENTRAL RING The core of an active galaxy is obscured by a ring of dust and gas that is dark on the 100 outside and bright within. MILLION DEGREES It is a powerful Celsius is the temperature that the source of core of a black hole can reach. energy.

ACCRETION DISK Formed by interstellar 1 2 3 gas and star remnants, the accretion disk can radiate X-rays because of The Force The Quasar Black Hole the extreme temperature of its center. of Gravity in the Core A black hole swallows the gas that begins to surround it. A hot, gaseous spiral forms, Gravitational force begins to The quasar in the core ejects two emitting high-speed jets. The magnetic field 4 unite vast quantities of hot, jets of particles that reach pours charged particles into the region around gaseous clouds. The clouds speeds approaching the speed of the black hole, and the exterior of the disk attract one another and collide, light. The quasar stage is thought absorbs interstellar gas. forming stars. A large amount of to have been the most violent PARTICLES Stable Galaxy gas accumulates at the center of stage in the formation of ejected from the black hole Nine billion years after its formation, with a the galaxy, intensifying galaxies. The gases and stars have intense magnetic fields. supermassive black hole at its core, the galaxy gravitational forces until a arising from the jets are The jets of particles travel at drastically slows its energetic activity, forming a low- massive black hole comes into introduced as spirals into the speeds approaching the speed of energy core. The stabilization of the galaxy allowed being in the galaxy's core. black hole, forming a type of light when they leave the core. the formation of stars and other heavenly bodies. accretion disk known as a quasar. 36 WHAT IS IN THE UNIVERSE? UNIVERSE 37

Large Central Region HOT GASES BRIGHT STARS Magellanic Cloud The hot gases originating Bright stars are Because the Milky Way is full of clouds of dust and rock from the surface of the born from gas that Stellar Metropolis particles, its center cannot be seen from outside the galaxy. central region may be the is not absorbed by The Milky Way's center can be seen only through telescopes result of violent explosions the black hole. Most MILKY WAY that record infrared light, radio waves, or X-rays, which can in the accretion disk. of them are young. or a long time, our galaxy (called the Milky Way because of pass through the material that blocks visible light. The central axis of the Milky Way contains ancient stars, some 14 billion years old, and exhibits intense activity within its BLACK HOLE its resemblance to a stream of milk in the night sky) was a Many astronomers believe that interior, where two clouds of hot gas have been found: F a black hole occupies the true enigma. It was Galileo Galilei who, in 1610, first pointed Sagittarius A and B. In the central region, but outside the center of the Milky Way. Its Andromeda core, a giant dark cloud contains 70 different types of strong gravitational force a telescope at the Milky Way and saw that the weak whitish strip molecules. These gas clouds are associated with violent Small Galaxy would trap gases in activity in the center of our galaxy and contain the heart of was composed of thousands and thousands of stars that appeared Magellanic orbit around it. Cloud Triangle the Milky Way within their depths. In general, the stars in to almost touch each other. Little by little, astronomers began to Galaxy this region are cold and range in color from red to orange. realize that all these stars, like our own Sun, were part of the enormous ensemble—the galaxy that is our stellar metropolis. SAGITTARIUS B2 MAGNETISM The largest dark cloud in The center of the Milky the central region of the Way is surrounded by Milky Way, Sagittarius B2 strong magnetic fields, contains enough alcohol to perhaps from a rotating cover the entire Earth. black hole. GASES SWIRL outward because of forces in the Structure of the Milky Way Sagittarius A region. Because the ROTATION gas rotates at high speed but The Milky Way, containing more than 100 billion stars, has two spiral arms The speeds of the rotation of the various parts of the remains concentrated, it could be rotating around its core. The Sagittarius arm, located between the Orion arm Milky Way vary according to those parts' distances from trapped by gravitational forces and the center of the Milky Way, holds one of the most luminous stars in the galaxy, the core of the galaxy. The greatest number of stars is exerted by a black hole. Eta Carinae. The Perseus arm, the main outer arm of the Milky Way, contains young concentrated in the region between the Milky Way's core and its border. Here the speed of rotation is much stars and nebulae. The Orion arm, extending between Perseus and Sagittarius, greater because of the attraction that the objects in this houses the solar system within its inner border. The Orion arm of the Milky region feel from the billions of stars within it. Way is a veritable star factory, where gaseous The Exact Center interstellar material can give birth to billions of stars. Remnants of The core of the Milky Way galaxy is marked galaxy. The speed of its rotation is an by very intense radio-wave activity that indication of the powerful gravitational stars can also be found might be produced by an accretion disk force exerted from the center of the Milky within it. 120 miles per hour (200 km/h) made up of incandescent gas surrounding a Way, a force stronger than would be massive black hole. The region of Sagittarius produced by the stars located in the region. 00 3600 A, discovered in 1994, is a gas ring that The hot, blue stars that shine in the center 140 miles per hour (220 km/h) rotates at very high speed, swirling within of the Milky Way may have been born from several light-years of the center of the gas not yet absorbed by the black hole. 300 150 miles per hour (240 km/h)

5 miles per hour (250 km/h) 15 A Diverse Galaxy

Central 0 600 270 protuberance The brightest portion of the Milky Way the Milky Way seem truly dark. The objects that that appears in photographs taken with can be found in the Milky Way are not all of one OUTER RING optical lenses (using visible light) is in the type. Some, such as those known as the halo A ring of dark clouds of dust and constellation Sagittarius, which appears to lie in population, are old and are distributed within a molecules that is expanding as a the direction of the center of the Milky Way. The sphere around the galaxy. Other objects form a result of a giant explosion. It is suspected that a small object in bright band in the nighttime sky is made up of more flattened structure called the disk the central region of the Milky stars so numerous that it is almost impossible to population. In the spiral arm population, we find Way might be its source. count them. In some cases, stars are obscured the youngest objects in the Milky Way. In these 900 by dense dust clouds that make some regions of arms, gas and interstellar dust abound.

2400 THE MILKY WAY IN VISIBLE LIGHT

Eta THE CONSTELLATION DARK REGIONS 1200 Carinae Eagle SAGITTARIUS Dark regions are Nebula Close to the center of produced by dense the Milky Way, clouds that obscure Sagittarius shines the light intensely. of stars.

SOLAR SYSTEM

Orion Nebula 2100 6,000 light-years 100,000 STARS Cassiopeia A So many LIGHT-YEARS stars The diameter of the Milky Way is SECTORS compose the Many different Milky Way that it large in comparison with other Crab sectors make up the is impossible for us Nebula galaxies but not gigantic. Milky Way. to distinguish them all. 1500

1800 OLYMPUS MONS, ON MARS ATTRACTED BY A STAR 40-41 JUPITER, GAS GIANT 50-51 PLUTO: NOW A DWARF 58-59 Olympus Mons is the largest volcano of the solar system. It A VERY WARM HEART 42-43 THE LORD OF THE RINGS 52-53 DISTANT WORLDS 60-61 is about two-and-a-half times The Solar System as high as Mount Everest. MERCURY, AN INFERNO 44-45 URANUS WITHOUT SECRETS 54-55 CONSTRUCTION DEBRIS: ASTEROIDS AND METEORITES 62-63 VENUS, OUR NEIGHBOR 46-47 NEPTUNE: DEEP BLUE 56-57 THOSE WITH A TAIL 64-65 RED AND FASCINATING 48-49

mong the millions and Sun. To ancient peoples, the Sun was a around it, make up the solar system, the most explored planet. Here we see a millions of stars that form the god; to us, it is the central source of which formed about 4.6 billion years ago. photo of Olympus Mons, the largest Milky Way galaxy, there is a energy that generates heat, helping life The planets that rotate around it do not volcano in the entire solar system. It is A medium-sized one located in exist. This star, together with the planets produce their own light. Instead, they almost two-and-a-half times as high as the one of the galaxy's arms—the and other bodies that spin in orbits reflect sunlight. After the Earth, Mars is tallest peak on the Earth, Mount Everest. 40 THE SOLAR SYSTEM UNIVERSE 41 Attracted by a Star lanets and their satellites, asteroids and other rocky interpretation, the planets complete elliptical trajectories, called objects, and an incalculable number of cometlike objects, orbits, around the Sun. In every case, the movement is produced P some more than 1 trillion miles (1.6 trillion km) from the by the influence of the gravitational field of the Sun. Today, as Sun, make up the solar system. In the 17th century, astronomer part of a rapidly developing field of astronomy, it is known that Johannes Kepler proposed a model to interpret the dynamic planet or planetlike bodies also orbit other stars. properties of the bodies of the solar system. According to this

ORBITS Earth's Venus's Mercury's Mars's Main orbit orbit orbit orbit belt Outer Planets In general, the planets orbit in one Planets located outside the asteroid belt. They are enormous gas spheres with common plane small solid cores. They have very low temperatures because of their great called the elliptic. distance from the Sun. The presence of ring systems is exclusive to these planets. The greatest of them is Jupiter: 1,300 Earths could fit inside of it. Its mass is 2.5 times as great as that of the rest of the planets combined.

NEPTUNE DIAMETER 30,775 MILES (49,528 KM) MOONS 13

Triton Proteus Nereid URANUS DIAMETER 31,763 MILES (51,118 KM) MOONS 27 Jupiter's Saturn's Uranus's Neptune's orbit orbit orbit orbit

Titania Oberon Umbriel Ariel Miranda Puck The rotation of most planets around their own axes is in counterclockwise direction. Venus and Uranus, however, revolve clockwise. SATURN DIAMETER 74,898 MILES (120,536 KM) MOONS 50+

Titan Rhea Iapetus Tethys

BUILDING PLANETS Early ideas suggested that the planets formed gradually, beginning with the binding of hot dust particles. Today scientists suggest that the planets originated from the JUPITER collision and melding of larger-sized bodies called planetesimals. DIAMETER 88,846 MILES (142,984 KM) MOONS 60+ 1 ORIGIN Remains from the formation of the Sun Ganymede Callisto Io Europa created a disk of gas and dust around it, from which the planetesimals formed.

2 COLLISION Through collisions among themselves, planetesimals of Asteroid Belt different sizes joined together to become The border between the outer and inner more massive objects. planets is marked by millions of rocky fragments of various sizes that form a band called the asteroid belt. Their orbits are 3 influenced by the gravitational pull exerted on HEAT them by the giant planet Jupiter. This effect also The collisions keeps them from merging and forming a planet. produced a large amount of heat that accumulated in the interior of the planets, according to their distance from the Sun.

Phobos Deimos MARS DIAMETER 4,217 MILES (6,786 KM) MOONS 2 Inner Planets SOLAR EARTH GRAVITY Planets located inside the asteroid MOON DIAMETER 7,926 MILES The gravitational pull of the belt. They are solid bodies in which Sun upon the planets not internal geologic phenomena, such as (12,756 KM) only keeps them inside volcanism, which can modify their surfaces, MOONS 1 the solar system but are produced. Almost all of them have an also influences the speed with which appreciable atmosphere of some degree of VENUS MERCURY they revolve in their thickness, according to individual orbits around the Sun. DIAMETER DIAMETER circumstances, which plays a key role in 7,520 MILES 3,031 MILES Those closest to the Sun the surface temperatures of each planet. (12,103 KM) (4,878 KM) revolve in their orbits much MOONS MOONS faster than those farther 0 0 from it.

SUN 42 THE SOLAR SYSTEM UNIVERSE 43

Surface and Atmosphere A Very Warm Heart The visible portion of the Sun is a sphere of toward its outermost region. Above the light, or photosphere, made of boiling gases photosphere lies the solar atmosphere—the he Sun at the center of the solar system is a source of light and emanating from the solar core. The gas flares form chromosphere and the corona. The energy plasma, which passes through this layer. Later the generated at the core moves through the surface of heat. This energy is produced by the fusion of atomic hydrogen nuclei, gas flares enter a vast gas layer called the solar the photosphere and solar atmosphere for T atmosphere. The density of this layer decreases thousands of years in search of an exit into space. which generate helium nuclei. The energy that emanates from the Sun travels through space and initially encounters the bodies that SUNSPOTS populate the solar system. The Sun shines thanks to thermonuclear are regions of gases that are PENUMBRA generally colder (7,232° F [4,000° Peripheral region. It is fusion, and it will continue to shine until its supply of hydrogen C]) than the photosphere (10,112° F the hottest and brightest runs out in about six or seven billion years. [5,600° C]). For that reason, they part of the Sun. appear dark.

UMBRA PHOTOSPHERE Central region. Very Gassy It is the The visible surface of the Sun, a boiling tide, coldest and darkest The Sun is a giant ball of gases with very high density and is thick with gases in a plasma state. In its uppermost layer, its density decreases and its part. temperature. Its main components are hydrogen (90%) and NUCLEAR COLLISION transparency increases, and the solar radiation helium (9%). The balance of its mass is made up of trace elements, Two hydrogen nuclei (two escapes from the Sun as light. The such as carbon, nitrogen, and oxygen, among others. Because of the 1. protons) collide and remain spectrographic study of this layer has allowed conditions of extreme temperature and pressure on the Sun, these joined. One changes into a scientists to confirm that the main components of CHROMOSPHERE elements are in a plasma state. neutron, and deuterium the Sun are hydrogen and helium. Above the photosphere, and of less density, forms, releasing a neutrino, lies the chromosphere, a layer 3,110 miles a positron, and 10,112º F (5,000 km) thick. Its temperature ranges 900,000º F CHARACTERISTICS CONVECTIVE ZONE a lot of energy. from 8,100° F (4,500° C) to 900,000° F (500,000º C) extends from the base of the (5,600º C) (500,000° C) with increasing altitude. The CONVENTIONAL photosphere down to a depth of temperature of the corona can reach MAXIMUM TEMPERATURE PLANET 15 percent of the solar radius. 1,800,000° F (1,000,000° C). OF THE CHROMOSPHERE SYMBOL Here energy is transported up toward the surface by gas currents (through convection). CORE SPICULES ESSENTIAL DATA The core occupies only 2 percent Vertical jets of gas that spew from the chromosphere, usually of the total volume of the Sun, reaching 6,200 miles (10,000 km) in height. They originate in Proton Average distance 93 million miles Positron but in it is concentrated about upper convection cells and can rise as high as the corona. from Earth (150 million km) half the total mass of the Sun. The great pressures and Equatorial 864,000 miles temperatures in the core produce diameter (1,391,000 km) RADIATIVE ZONE This portion of the Sun is Neutron thermonuclear fusion. Orbital 7,456 miles per traversed by particles coming Neutrino speed second (12,000 km/s) from the core. A proton can take a million years to cross this zone. Mass* 332,900 MACROSPICULES This type of vertical eruption is similar to a Gravity* 28 27,000,000º F spicule, but it usually reaches up to 25,000 Density 0.81 ounce per cubic miles (40,000 km) in height. inch (1.4 g per cu cm) 14,400,000º F Deuterium (15,000,000º C) Average 9,932° F temperature (8,000,000º C) (5,500° C) CORONA Atmosphere Dense Located above the Moons chromosphere, it extends None millions of miles into space and *In both cases, Earth = 1 reaches temperatures nearing Photon 1,800,000° F (1,000,000° C). 1,800,000º F It has some holes, or low- (1,000,000º C) density regions, through which NUCLEAR FUSION THE TEMPERATURE IN THE CORONA gases flow into the solar wind. OF HYDROGEN Deuterium 1 PHOTONS The extraordinary temperature of The deuterium formed the nuclear core helps the hydrogen 2. collides with a proton. nuclei join. Under conditions of This collision releases one SOLAR WIND lower energy, they repel each other, photon and gamma rays. Consists of a flux of ions emitted by the solar but the conditions at the center of The high-energy photon atmosphere. The composition is similar to that of the Sun can overcome the repulsive needs 30,000 years to the corona. The Sun loses approximately 1,800 HELIUM pounds (800 kg) of matter per second in the forces, and nuclear fusion occurs. reach the photosphere. SOLAR PROMINENCES NUCLEUS Clouds and layers of gas from form of solar wind. For every four hydrogen nuclei, a the chromosphere travel series of nuclear reactions produce thousands of miles until they one helium nucleus. reach the corona, where the Deuterium 2 influence of magnetic fields causes them to take on the shape of an arc or wave. HELIUM NUCLEI The group of two protons and a neutron collides with 3. SOLAR FLARES another such group. A These eruptions come out of the solar helium nucleus forms, and a atmosphere and can interfere with radio pair of protons is released. Proton 1 Proton 2 communications on Earth. 44 THE SOLAR SYSTEM UNIVERSE 45

Mercury, an Inferno Composition and Magnetic Field Like the Earth, Mercury has a magnetic field, although a much 29% 22% EXTREMELY THIN ATMOSPHERE weaker one. The magnetism results from its enormous core Mercury's atmosphere is almost nonexistent and consists Sodium Hydrogen of a very thin layer that cannot protect the planet either ercury is the planet nearest to the Sun and is therefore the one that has to made of solid iron. The mantle that surrounds the core is believed to from the Sun or from meteorites. During the day, when be a fine layer of iron and sulfur. Mercury is closer to the Sun, the planet's temperature can withstand the harshest of the Sun's effects. Due to its proximity to the Sun, Mercury 6% surpass 842° F (450° C). At night, temperatures can M Helium plummet to -297° F (-183° C). moves at great speed in its solar orbit, completing an orbit every 88 days. It has almost no

atmosphere, and its surface is dry and rugged, covered with craters caused by the impact of During the night, the During the 43% heat of Mercury's day, the Sun Others numerous meteorites; this makes it resemble the Moon. Numerous faults, formed during the cooling of CRUST rocks is lost rapidly, directly heats Made of silicate rocks, and the planet's the rock. the planet when it was young, are also visible on the surface. Constantly baked by its neighbor, the Mercury's crust is similar temperature drops. Sun, Mercury has an average surface temperature of 333° F (167° C). to the crust and mantle of the Earth. It has a thickness of 310 to 370 -297º F 883º F miles (500-600 km). 310 miles (500 Km) (-183º C) (473º C)

A Scar-Covered Surface CHARACTERISTICS The surface of Mercury is very similar to that of the Moon. It is possible to find CORE craters of varying sizes. The largest one has a diameter of some 810 miles (1,300 CONVENTIONAL Dense, large, and made PLANET SYMBOL km). There are also hills and valleys. In 1991, radio telescopes were able to detect of iron, its diameter possible evidence of the presence of frozen water in Mercury's polar regions, may be as great as 2,240 miles 2,240 to 2,300 miles ESSENTIAL DATA information that Mariner 10 had been unable to gather. Mariner 10, the only (3,600 Km) (3,600-3,800 km). mission sent to Mercury, flew by the planet three times between 1974 and 1975. Average distance 36,000,000 miles The polar ice was found at the bottom of very deep craters, which limit the from the Sun (57,900,000 km) ice's exposure to the Sun's rays. The spacecraft Messenger, launched in Solar orbit 88 days 2004, is scheduled to orbit the planet Mercury in 2011 and is expected to (Mercurian year) 00 hours provide new information about Mercury's surface and magnetic field. Equatorial 3,032 miles diameter (4,880 km) Orbital speed 29.75 miles per second (47.87 km/s) CALORIS CRATER The largest impact crater in the Mass* 0.06 solar system, it has a diameter Gravity* 0.38 of 810 miles (1,300 km). Density 3.14 ounces per cubic inch (5.43 g/cu cm) The crater was Average temperature 332° F (167° C) flooded Atmosphere with lava. Almost nonexistent Lunas Baked by its neighbor the Sun, Mercury is the planet with *In both cases, Earth = 1 When the projectile that formed the MANTLE the greatest thermal fluctuations in the solar system. Its 333º F AXIS INCLINATION crater struck, Mercury was still Made up mostly of average temperature is 333° F (167° C), but when it gets forming. The extensive waves that silica-based rocks (167º C) closer to the Sun, the temperature can climb to 842° F extended from the site of impact (450° C). At night, it drops to -297° F (-183° C). 0.1° formed hills and mountains ranges. One rotation lasts 59 days. Rotation and Orbit BEETHOVEN Mercury rotates slowly on its axis and takes approximately 59 sunrises. A person observing the sunrise from position 1 would have to is the second largest crater on Earth days to complete a turn, but it only needs 88 days to wait for the planet to make two orbits around the Sun and make three Mercury. It is 400 miles (643 travel in its orbit. To an observer in Mercury, these two combined rotations on its own axis before seeing the next sunrise. km) in diameter. Its floor was flooded by lava and later motions would give a combined interval of 176 days between two marked by meteorite impacts. ORBIT OF MERCURY VIEW FROM AROUND THE SUN MERCURY

3 2 Reaches the zenith Missions to Mercury 3 (noon) and stops 4 The space probe Mariner 10 was the first to reach Mercury. Between Resumes its original 1974 and 1975, the craft flew by the planet three times and came 1 6 path toward the horizon within about 200 miles (320 km) of the surface. Messenger, a space probe SUN scheduled to study Mercury between 2008 and 2011, was launched in 2004. 5 Recedes Stops 4 a bit 5 again Rises and 6 7 Mariner 10 Messenger 2 increases its size Each number The probe will pass corresponds to a position Decreases toward The Sun by Mercury twice in of the Sun in the sky as 7 the sunset 2008 and once seen from Mercury. 1 rises. again in 2009 before beginning to orbit the planet. HORIZON OF MERCURY 46 THE SOLAR SYSTEM UNIVERSE 47

VENUS'S PHASES VENUS'S PHASES WAXING FIRST WAXING WANING LAST WANING Venus, Our Neighbor AS SEEN FROM CRESCENT QUARTER GIBBOUS GIBBOUS QUARTER CRESCENT As Venus revolves around the EARTH Sun, its solar illumination varies enus is the second closest planet to the Sun. Similar in size to the Earth, it as is seen from the Earth has a volcanic surface, as well as a hostile atmosphere governed by the effects of carbon depending upon its position in V relation to the Sun and the dioxide. Although about four billion years ago the atmospheres of the Earth and Venus were Earth. Thus Venus has phases THE NEW AND FULL similar, the mass of Venus's atmosphere today is 100 times greater than the Earth's. Its thick similar to the Moon's. During its SUN PHASES ARE NOT elongations, when Venus is VISIBLE FROM EARTH. clouds of sulfuric acid and dust are so dense that stars are invisible from the planet's surface. farthest from the Sun in the sky, EARTH VENUS Viewed from the Earth, Venus can be bright enough to be visible during day and second only to the Venus appears at its brightest. moon in brightness at night. Because of this, the movements of Venus were well-known by most ancient civilizations. Surface CHARACTERISTICS Composition The Venusian surface has not remained the same throughout its life. The current one is some 500 million years old, but the rocky landscape visible CONVENTIONAL The overwhelming presence of carbon dioxide in the farther from the Sun and reflects all but 20 percent of the Sun's The surface of Venus is rocky and dry. Most of today was formed by intense volcanic activity. Volcanic rock covers 85 PLANET SYMBOL Venusian atmosphere induces a greenhouse effect, light. The surface temperature of Venus is relatively constant, the planet is formed by percent of the planet. The entire planet is crisscrossed by vast plains increasing the surface temperature to 864° F (462° C). Because averaging 860° F (460° C). The atmospheric pressure on Venus is ESSENTIAL DATA volcanic plains and and enormous rivers of lava, as well as a number of mountains. The of this, Venus is hotter than Mercury, even though Venus is 90 times greater than that on the Earth. other, elevated regions. Average distance lava flows have created a great number of grooves, some of which 67,000,000 miles 3,700 miles from the Sun (108,000,000 km) (6,000 km) are very wide. The brightness of Venus's surface is the result of Solar orbit 224 days ATMOSPHERE metallic compounds. (Venusian year) MANTLE CORE 17 hours Venus's glowing appearance 50 miles Made of molten rock, it It is believed that Venus's core is Equatorial 7,520 miles is caused by the planet's thick, constitutes most of the similar to the Earth's, containing diameter (12,100 km) suffocating atmosphere, (80 km) planet. It traps the solar metallic elements (iron and which is made up of carbon radiation and is between nickel) and silicates. Venus has no Orbital speed 22 miles per second dioxide and sulfuric clouds IS THE THICKNESS OF 37 and 62 miles (60 and magnetic field—possibly because MAGELLAN (35 km/s) that reflect sunlight. THE ATMOSPHERE. 100 km) thick. of its slow axial rotation. Venus was explored by the Mass* 0.8 Magellan spacecraft Gravity* 0.9 between 1990 and 1994. The probe was equipped with a Density 3.03 ounces per cubic 14,400º F radar system to observe the inch (5.25 g/cu cm) Carbon Nitrogen and surface through its dense Average temperature dioxide traces of (8,000º C) atmosphere. 860° F (460° C) other gases Atmosphere Very thick 3,700 miles (6,000 km) Moons None 97% 3% *In both cases, Earth = 1

AXIS INCLINATION 117° Rotates on its own axis every 243 days

GREENHOUSE EFFECT Only 20 percent of the Sun's light reaches the surface of Venus. The thick clouds of dust, sulfuric acid, and carbon dioxide that constitute Venus's ISHTAR TERRA One of the raised plateaus of Venus, it atmosphere reflect the is similar in size to Australia and is remaining light, leaving Venus located close to Venus's north pole. It in permanent darkness. has four main rocky mountain ranges called Maxwell Montes, Freyja Montes, Akna Montes, and Dam Montes.

SOLAR RADIATION Venus is kept hot by its thick SULFURIC atmosphere, which retains the ACID energy of the Sun's rays.

864º F APHRODITE TERRA Larger than Ishtar Terra, it is the size Venus lacks water. A U.S. of South America. Aphrodite Terra robot probe sent to Venus in CRUST (462º C) lies near the equator and consists 1978 found some evidence Made up of mostly of mountainous regions to the that water vapor could have silicates, it is INFRARED RAYS thicker than the east and west, which are separated The surface of Venus radiates existed in the atmosphere Earth's crust. by a low-lying region. infrared radiation. Only 20 percent hundreds of millions of years of the Sun's rays pass through ago, but today no trace of Venus's thick clouds of sulfuric acid. water remains. 48 THE SOLAR SYSTEM UNIVERSE 49

OLYMPUS MONS Surface This gigantic, inactive volcano is not only the Red and Fascinating largest on Mars but also in the solar system. It is a place of geologic extremes, shaped by volcanic activity, meteorite bombardment, EVEREST OLYMPUS ars is the fourth planet from the Sun. Of all the planets, Mars most windstorms, and floods (though there is little or no 29,000 FEET 72,200 FEET water on Mars today). Mountains dominate the (8,848 METERS) (22,000 METERS) closely resembles the Earth. It has polar ice caps, and the tilt of its axis, southern hemisphere, but lowlands are common in M the northern hemisphere. period of rotation, and internal structure are similar to those of the Earth. Known as the Red Planet because of the reddish iron oxide that covers its surface, Mars has a thin atmosphere composed essentially of carbon dioxide. Mars does not have water, though it did in the past, and there is evidence some water might exist underground. Many spacecraft have been sent to explore

Mars, in part because it is the planet other than Earth most likely to have developed some form of 1,000 miles life, and it will probably be the first planet humans leave the Earth to visit. (1,700 km)

MANTLE Martian Orbit Composition It is made of -220º F molten rock of Because Mars's orbit is more (-140º C) Mars, a rocky planet, has an iron-rich core. Mars greater density VALLES MARINERIS elliptical than that of Earth, is almost half the size of the Earth and has a than the Earth's IN WINTER mantle. The canyon system of the Mars's distance from the Sun varies similar period of rotation, as well as clearly evident Valles Marineris was likely widely. At its perihelion, or closest clouds, winds, and other weather phenomena. Its thin caused naturally, primarily EARTH MARS approach to the Sun, Mars receives atmosphere is made up of carbon dioxide, and its red by water erosion. SUN 45 percent more solar radiation than 62º F color comes from its soil, which is rich in iron oxide. Olympus at its aphelion, or farthest point. Mons Temperatures on Mars range from (17º C) CHARACTERISTICS -220° F to 62° F (-140° C to 17° C). IN SUMMER CORE Small and likely CRUST composed of iron is thin and made up of CONVENTIONAL solid rock. It is 31 ns PLANET SYMBOL 2,000 miles o miles (50 km) thick. (3,294 km) s M rsi Tha ESSENTIAL DATA Moons Average 141,600,000 distance from miles s i the Sun (227,900,000 km) Mars has two moons, Phobos and Deimos. hours at an altitude of 14,627 miles (23,540 km), er in Both have a lower density than Mars and and Phobos orbits Mars in eight hours at an r Solar orbit 1.88 years a (Martian year) are pitted with craters. Phobos has a diameter of altitude of 5,840 miles (9,400 km). Astronomers M s e Equatorial 17 miles (27 km), and Deimos has a diameter of believe that the moons are asteroids that l 4,222 miles l diameter nine miles (15 km). Deimos orbits Mars in 30 were captured by Mars's gravity. a (6,794 km) V Orbital 15 miles per speed second (24 km/s) PHOBOS DEIMOS Mass* 0.107 Gravity* 0.38 is Lacus Density Sol 2.27 ounces per cubic ATMOSPHERE inch (3.93 g/cu cm) Thin and continuously Average thinning as solar winds -81° F temperature (-63° C) DIAMETER 9 MILES (15 KM) DIAMETER 17 MILES diminish atmosphere DISTANCE 14,627 MILES (27 KM) Atmosphere Very thin FROM MARS (23,540 KM) DISTANCE 5,840 MILES 95.3% Moons 2 FROM MARS (9,400 KM) Carbon *In both cases, Earth = 1 dioxide AXIS INCLINATION e Pol 2.6% outh S 25.2° Nitrogen num Sire One rotation MISSIONS TO MARS ra lasts 1.88 years. er Oxygen, carbon monoxide, After our own Moon, Mars has been a more attractive target for T 2.1% water vapor, and other gases exploratory missions than any other object in the solar system.

1965 MARINER 4 1969 MARINER 6 1971 MARINER 9 1973 MARS 4, MARS 5, 1976 VIKING 1 AND 2 1997 MARS 1997 MARS GLOBAL 2001 MARS ODYSSEY 2003 MARS EXPRESS 2004 SPIRIT AND 2006 MARS RECONNAISSANCE The first mission sent AND 7 studied the photographed the MARS 6, AND MARS 7 searched for traces of life. PATHFINDER was SURVEYOR took more mapped the mineralogy Orbiting probe. First OPPORTUNITY ORBITER made a detailed study to Mars, it made only southern hemisphere Olympus volcano Russian spacecraft They were the first spacecraft the third successful than 100,000 photos and morphology of spacecraft sent by the surveyed many square of the Martian surface while brief flyovers. and equator of Mars. for the first time. successfully sent to Mars to land on Martian soil. Mars landing. of the planet. Mars's surface. European Space Agency. miles of the surface. orbiting the planet. 50 THE SOLAR SYSTEM UNIVERSE 51

The Moons of Jupiter GALILEAN MOONS Jupiter, Gas Giant Jupiter has more than 60 moons. Many of not even have names. Jupiter's rotation is gradually Of Jupiter's 63 moons, four are visible from Earth with binoculars. These are called the Galilean moons in honor them have not been officially confirmed and do slowing because of the moons' tidal effects. of their discoverer, Galileo Galilei. Astronomers believe that Io has active volcanoes and that Europa has an upiter is the largest planet in the solar system. Its diameter is 11 times that ocean underneath its icy crust. of the Earth, and its mass is 300 times as great. Because the speed of Jupiter's rotation AMALTHEA THEBE IO GANYMEDE EUROPA GANYMEDE J ADRASTEA EUROPA flattens the planet at its poles, its equatorial diameter is greater than its polar diameter. METIS 2,000 MILES 3,270 MILES Jupiter rotates at 25,000 miles per hour (40,000 km/hr). One of the most distinctive elements (3,200 KM) (5,268 KM) of Jupiter's atmosphere is its so-called Great Red Spot, a giant high-pressure region of turbulence that has been observed from the Earth for more than 300 years. The planet is orbited by numerous satellites and has RADIUS 38,470 MILES 1 radius 23 4 56789 15 a wide, faint ring of particles. (61,911 KM) Enlarged region IO CALISTO CALLISTO LEDA HIMALIA LYSITHEA ELARA ANANKE CARME PASIPHAE SINOPE 2,264 MILES 2,986 MILES (3,643 KM) (4,806 KM)

23,000 miles 26 160/63/67 302 322 335/8 Composition (37,000 km) Jupiter is a giant ball of hydrogen and helium that have been compressed into liquid in the planet's ATMOSPHERE interior and into metallic rock measures 620 16,160 in its core. Not much is known miles (1,000 km). Winds about Jupiter's core, but it is The winds on Jupiter blow in contiguous bands believed to be bigger than the and opposing directions. The bands' small miles Earth's core. differences in temperature and chemical composition (26,000 km) give the planet its multicolored appearance. Jupiter's GREAT RED SPOT inclement environment, in which winds blow at more INNER MANTLE than 370 miles per hour (600 km/h), can cause large Surrounds the core. It is made of storms, such as the Great Red Spot in the southern liquid metallic hydrogen, an element hemisphere of the planet. The Great Red Spot, which is only found under hot, high-pressure 16,155 miles (26,000 km) long, is believed to be conditions. The inner mantle is a 17,000 miles (27,000 km) soup of electrons and nuclei. composed mainly of ammonia gas and clouds of ice.

RINGS Jupiter's rings are made of dust from the planet's four inner moons. These rings were first seen in 1979 by the CHARACTERISTICS space probe Voyager 1 and later by Voyager 2.

CONVENTIONAL PLANET SYMBOL CORE OUTER GOSSAMER RING Its size is similar to that of the Earth's core. ESSENTIAL DATA INNER GOSSAMER RING Average 483,000,000 distance from miles MAIN RING RING MATERIAL the Sun (778,000,000 km) 9,000 miles (14,000 km) HALO Solar orbit 11 years (Jovian year) 312 days Equatorial 88,700 miles JUPITER'S MAGNETISM diameter (142,800 km) Jupiter's magnetic field is 20,000 times stronger is surrounded by a huge magnetic bubble, the Orbital speed 8 miles per magnetosphere. The magnetosphere's tail reaches second (13 km/s) than the Earth's. Astronomers believe the field is caused by the electrical currents that are created more than 370,000,000 miles (600,000,000 Mass* 318 by the rapid rotation of metallic hydrogen. Jupiter km)—beyond the orbit of Saturn. Gravity* 2.36 ATMOSPHERE Density 0.77 ounce per cubic 54,000º F surrounds the inner inch (1.33 g/cu cm) liquid layers and the Jupiter's magnetosphere is Average -184° F (30,000º C) solid core. It is 620 the largest object in the solar temperature (-120° C) miles thick (1,000 km). system. It varies in size and Atmosphere Very dense OUTER MANTLE shape in response to the solar Made of liquid wind, which is composed of Moons More than 60 89.8% molecular hydrogen. the particles continuously *In both cases, Earth = 1 Hydrogen The outer mantle radiated from the Sun. merges with the AXIS INCLINATION atmosphere. 10.2% 3.1° Helium 400,000,000 One rotation With traces miles lasts 9 hours of methane and 55 minutes. and ammonia (650,000,000 km) 52 THE SOLAR SYSTEM UNIVERSE 53

The Lord of the Rings Surface Gaseous Exterior CHARACTERISTICS Like Jupiter, Saturn HAZE Saturn and Jupiter differ very little in their composition. Both CONVENTIONAL has a surface of clouds are gaseous balls surrounding solid cores. What sets Saturn PLANET SYMBOL aturn is the solar system's second largest planet. Like Jupiter, it is a large that form bands because of apart are its rings, formed by clustered pieces of ice that range in the planet's rotation. WHITE size from small particles to large chunks. Each particle in a ring is a ESSENTIAL DATA ball of gas surrounding a small, solid core. Saturn was the most distant planet CLOUDS Saturn's clouds are less satellite orbiting Saturn. From the Earth, the massed debris seems Average distance 887,000,000 miles S turbulent and less colorful to form large structures, but each discrete piece actually has its from the Sun (1,427,000,000 km) discovered before the invention of the telescope. To the naked eye, it looks like a yellowish star, but than Jupiter's. The higher, own orbit. Solar orbit 29 years with the help of a telescope, its rings are clearly visible. Ten times farther from the Sun than the Earth, white clouds reach DEEP AND (Saturnine year) 154 days ORANGE CLOUDS Saturn is the least dense planet. If an ocean could be found large enough to hold it, Saturn would float. temperatures of -220° F ATMOSPHERE Equatorial 74,940 miles (-140° C). A layer of haze COMPONENTS diameter (120,600 km) BLUISH The main extends above the clouds. <1% 2% Orbital speed 6 miles per second CLOUDS 97% components of Sulfur gives it a (10 km/s) Helium Hydrogen Saturn's atmosphere yellowish appearance. Rings are hydrogen (97%) Mass* 95 and helium (2%). Gravity* 0.92 The rest is composed Saturn's rings, the brightest rings in the solar Density 0.4 ounce per cubic ENCKE DIVISION CASSINI DIVISION D RING of sulfur, methane, system, are made of rock and ice and orbit A small gap that separates 3,100 miles (5,000 km) wide, it is The closest ring to the inch (0.7 g/cu cm) and other gases. Saturn's equator. The rings are probably remains of ring A into two parts located between the A and B rings. surface of Saturn—so WINDS Saturn's winds generally Average temperature -193° F (-125° C) destroyed comets that were trapped by Saturn's near that it almost F RING A RING B RING touches the planet reach speeds of about 220 Atmosphere Very dense gravitational field. The farthest Saturn's Saturn's miles per hour (360 km/h), Moons More than 45 visible ring outer ring brightest and C RING widest ring Saturn's only causing strong storms. *In both cases, Earth = 1 RINGS G transparent AXIS INCLINATION AND E ring 310 miles (500 km) 26.7° 9,100 miles One rotation (14,600 km) lasts 10 hours and 39 minutes.

15,500 miles (25,000 km) 15,800 miles (25,500 km) OUTER MANTLE This layer is formed by liquid molecular hydrogen.

10,900 miles (17,500 km) ATMOSPHERE 5,300 miles Mainly hydrogen (8,500 km) and helium 2,200 miles (3,500 km)

THICKNESS AND WIDTH Although Saturn's INNER MANTLE It is made up of liquid rings are very metallic hydrogen. wide, their thickness is sometimes less 9,300 miles than 33 feet (15,000 km) (10 m).

The Moons of Saturn Saturn has more than 45 moons, making Saturn's family of moons one of the largest in the solar system. The sizes of the moons vary from Titan's 3,200 miles (5,150 km) CORE to tiny Calypso's 10 miles (16 km). 9,300 miles Composed of rock and (15,000 km) metallic elements, such as silicates and iron PANDORA EPIMETHEUS ENCELADUS POLYDEUCES PROMETHEUS JANUS TETHYS RHEA TITAN ATLAS MIMAS TELESTO 3,200 MILES 21,600ºF CALYPSO PAN METHONE PALLENE DIONE (5,150 KM) DAPHNIS HELENE DIAMETER (12,000º C)

RADIUS = 37,500 MILES 1 radius 23 4 5678 20 (60,300 KM)

Enlarged Titan has a larger region diameter than HYPERIONIAPETUS PHOEBE Mercury. It has an atmosphere that is mostly made of 25 61 220 nitrogen. 54 THE SOLAR SYSTEM UNIVERSE 55

EPSILON Rings LAMBDA Uranus Without Secrets Like all giant planets of the solar system, Uranus has a ring DELTA system, but it is much darker than Saturn's and more difficult GAMMA to see. The planet's 11 rings, which orbit the planet's equator, were o the unaided eye, Uranus looks like a star at the limit of visibility. It is the discovered in 1977. In 1986, they were explored by Voyager 2. seventh farthest planet from the Sun and the third largest planet in the solar system. ETA T BETA One peculiarity distinguishing it from the other planets is its anomalous axis of rotation, tilted nearly 98 degrees around the plane of its orbit, so that one or the other of ALPHA Uranus's poles points toward the Sun. Astronomers speculate that, during its formation, Uranus may have suffered an impact with a protoplanet, which could have altered Uranus's Satellites tilt. Uranus's orbit is so large that the planet takes 84 years to completely orbit the Sun. 4 Uranus has 27 moons. The first four were discovered Shakespeare and Alexander Pope, a naming convention that in 1787, and another 10 were identified in 1986 by distinguishes them from the other moons in the solar Uranus's period of rotation is 17 hours and 14 minutes. 5 the space probe Voyager 2. Uranus's moons were named in system. Some of Uranus's moons are large, but most 6 honor of characters from the works of William measure only dozens of miles. 1986U2R CRESSIDA DESDEMONA MIRANDA ARIEL UMBRIEL OBERON BIANCA JULIET MAGNETIC FIELD OPHELIA PORTIA TITANIA Uranus generates a magnetic Magnetopause Some scientists suggest CORDELIA ROSALIND field 50 times more powerful that Uranus's anomalous BELINDA than Earth's. This field is not magnetic field may indicate that the convection of 6,200 MILES PUCK centered on the planet, but is Cusp (10,000 KM) offset and tilted 60 degrees Uranus's core has stopped from Uranus's axis. If this were Capture region because of cooling—or, the case on Earth, the magnetic perhaps, that the planet is RADIUS= north pole would be located in currently undergoing a 15,882 MILES 1 radius 23 4 56789 1621 Morocco. Unlike other planets, magnetic inversion, as (25,559 KM) Uranus's magnetic field has happened on the Enlarged originates in the planet's mantle, Magnetic Earth. region 2001U3 2003U3 not its core. envelope (FRANCISCO) CALIBAN STEPHANO TRINCULO SYCORAX (MARGARET) PROSPERO SETEBOS

10,600 (17,000MILES KM) 169 283 314 339 482 580 654 698 Composition

Uranus's core is made of CORE abundant amounts of silicates Made up TITANIA and ice. The planet is almost four of silicates MOONS 980 MILES times larger than the Earth, and its and ice Uranus has small, dark moons, discovered by Voyager 2, as well as bigger (1,578 KM) atmosphere is made up of hydrogen, moons, such as Miranda, Ariel, Umbriel, Oberon, and Titania. These last two are approximately 930 miles (1,500 km) in diameter. helium, and methane. Uranus has an almost horizontal tilt, causing it to 6,200 MILES have very long seasons. (10,000 KM) Miranda, only 293 miles UMBRIEL (472 km) in diameter, is 730 MILES the smallest of Uranus's (1,170 KM) CHARACTERISTICS INNER MANTLE five main moons. It has CONVENTIONAL Probably icy water, methane, an irregular surface PLANET and ammonia. (According to with grooves and a SYMBOL some models, the materials of bright mark. the mantle and core do not ESSENTIAL DATA form layers.) Average distance 1,780,000,000 miles MIRANDA from the Sun (2,870,000,000 km) 293 MILES Solar orbit 84 years (472 KM) (Uranian year) 4 days Equatorial 32,200 miles OBERON diameter (51,800 km) ARIEL 946 MILES Orbital speed 4 miles per OUTER MANTLE 720 MILES (1,522 KM) Composed primarily of second (7 km/s) (1,158 KM) hydrogen and helium, as well Mass* 14.5 as a small amount of methane Gravity* 0.89 Density 0.8 ounce per cubic inch (1.3 g/cu cm) ATMOSPHERE Surface REFRACTION OF RAYS Average temperature -346° F (-210° C) Uranus's atmosphere is made up of hydrogen, methane, helium, For a long time, Uranus Atmosphere When sunlight passes through this layer, the methane Less dense and small amounts of acetylene In Uranus, sunlight is reflected by a was believed to have a absorbs the red light waves and lets the blue light Moons 27 and other hydrocarbons. curtain of clouds that lie underneath smooth surface. The Hubble a layer of methane. waves pass through, producing the planet's hue. * In both cases, Earth = 1 1. 2. Space Telescope, however,

AXIS INCLINATION showed that Uranus is a ATMOSPHERE ATMOSPHERE 85% dynamic planet that has the SUNLIGHT SUNLIGHT 97.9° -346° F Hydrogen solar system's brightest URANUS URANUS One rotation clouds and a fragile ring lasts 17 hours (-210° C) 3% 12% system that wobbles like an and 14 minutes. AVERAGE TEMPERATURE Methane Helium unbalanced wheel. 56 THE SOLAR SYSTEM UNIVERSE 57

Surface Ascending THE GREAT SPOT This giant storm, called Neptune: Deep Blue White methane clouds surround Neptune, winds the Great Dark Spot, was circulating at some of the fastest speeds in first seen on the surface een from our planet, Neptune appears as a faint, blue point invisible to the solar system. Neptune's winds reach 1,200 of Neptune in 1989 and miles per hour (2,000 km/h) from east to west, was as large as the Earth. swirling against the direction of the planet's By 1994, it had the naked eye. Images sent to Earth by Voyager 2 show the planet as a disappeared. S rotation. remarkably blue sphere, an effect produced by the presence of methane in the outer part of Neptune's atmosphere. The farthest of the gaseous planets, Neptune is 30 times farther from the Sun than the Earth is. Its rings and impressive clouds are noteworthy, as is its resemblance to Uranus. Neptune is of special interest to astronomers because, before its discovery, its Descending existence and location were predicted on the basis of mathematical calculations. winds

Moons Neptune has 13 natural satellites, nine of which are were observed from space by the U.S. space probe Voyager 2. named. Triton and Nereid were the first moons All the names of Neptune's satellites correspond to ancient observed by telescope from Earth. The 11 remaining moons Greek marine deities. Hard Heart According to some models, Neptune has a rocky silicate core, covered by a mantle of icy water, ammonia, hydrogen, and methane. According to some THALASSA DESPINA PROTEUS TRITON NEREID models, however, the materials of the mantle and core do not form layers.

NAIAD GALATEA LARISSA

NEPTUNE'S RADIUS= 15,388 MILES 1 radius 23456 7 8 9 10 11 12 13 14 222 CORE 3,700 miles Made up of (24,764 KM) (6,000 km) silicates and ice

TRITON Its diameter is 1,681 miles (2,706 km). Triton orbits -391° F Neptune in a direction opposite that of the other moons. Its surface has dark (-235° C) stripes formed by the is its temperature, making miles material spewed from its Triton one of the coldest 1,200 CHARACTERISTICS geysers and volcanoes. bodies in the solar system. CONVENTIONAL 8,700 miles per hour (14,000 km) INNER MANTLE PLANET Probably icy water, SYMBOL (2,000 km/h) methane, and ammonia GALLE ESSENTIAL DATA Rings Average distance 2,800,000,000 miles from the Sun (4,500,000,000 km) Uranus has faint rings of dust. When they OUTER MANTLE Solar orbit 164 years were discovered from the Earth, Composed primarily of (Neptunian year) 264 days hydrogen and helium, as well astronomers thought the rings formed Equatorial as a small amount of methane 30,800 miles incomplete arcs. The ring names LE VERRIER diameter (49,500 km) honor the first scientists LASSELL Orbital speed 3.4 miles per second to study Neptune. (5.5 km/s) ATMOSPHERE Banded, like the atmospheres Mass* 17.2 of the other gas giants, Gravity* 1.12 Neptune's atmosphere forms a Density 1 ounce per cubic inch cloud system at least as active (1.6 g/cu cm) 4,500 miles (7,200 km) as Jupiter's. Average temperature ARAGO -330° F (-200° C) Atmosphere Dense COMPOSITION Moons 13 Neptune's rings are dark, *In both cases, Earth = 1 like those of Uranus and 89.8% Jupiter. Their composition is unknown, and they are Hydrogen AXIS INCLINATION ADAMS believed to be unstable. 28.3° Located 39,000 miles (63,000 km) from Liberty, which makes up 10.2% the planet's core, this ring has three part of the outer ring, One rotation lasts prominent arcs, or sections, named could vanish before the Helium 16 hours and 36 Liberty, Fraternity, and Equality. 22nd century. minutes. 58 THE SOLAR SYSTEM UNIVERSE 59

Composition CHARACTERISTICS Pluto: Now a Dwarf CONVENTIONAL Scientific calculations have deduced that 75 Pluto is an object that belongs to the Kuiper belt, a group PLANET SYMBOL percent of Pluto consists of a mixture of rocks and of objects left over from the formation of the outer ice. This frozen surface is made up of 98 percent planets. In addition to large amounts of frozen nitrogen, ESSENTIAL DATA Pluto stopped being the ninth planet of the solar system in 2006 when Average distance nitrogen, as well as traces of solidified carbon monoxide Pluto has simple molecules containing hydrogen and 3,700,000,000 miles from the Sun (5,900,000,000 km) the International Astronomical Union decided to change the classification of cold, distant Pluto to and methane. Recently scientists have concluded that oxygen, the building blocks of life. P that of dwarf planet. This tiny body in our solar system has never had an imposing profile, and it Solar orbit (Plutonian year) 247.9 years has not yet been possible to study it closely. All that is known about Pluto comes through observations ATMOSPHERE Equatorial 1,400 miles Pluto's very thin atmosphere freezes and diameter (2,247 km) made from the Earth or Earth orbit, such as those made by the Hubble Space Telescope. Despite the lack falls to the dwarf planet's surface as Orbital speed 3 Pluto moves toward its aphelion. CRUST miles per second of information gathered about Pluto, it is notable for its unique orbit, the tilt of its axis, and its location The crust of this (4.8 km/s) dwarf planet is Mass* 0.002 within the Kuiper belt. All these characteristics make Pluto especially intriguing. made of methane Gravity* 0.067 98% and water frozen Density 1.2 ounces per cubic inch Nitrogen on the surface. (2.05 g/cu cm) Average temperature -380° F (-230° C) 2% Atmosphere Very thin PLUTO Surface Methane Moons 3 A Double World With some * In both cases, Earth = 1 Only a little is known about Pluto, traces of carbon Pluto and its largest satellite, ROTATION but the Hubble Space Telescope monoxide AXIS INCLINATION Charon, have a very special AXIS showed a surface covered by a frozen relationship. They have been called mixture of nitrogen and methane. The 122° double planets—the diameter of Charon presence of solid methane indicates that One rotation is about that of Pluto. One theory its temperature is less than -333° F lasts 6.387 hypothesizes that Charon was formed (-203° C), but the dwarf planet's Earth days. from ice that was torn from Pluto when temperature varies according to its another object collided with the dwarf place in orbit, ranging between 30 and planet. CHARON 50 astronomical units from the Sun. 270 miles (434 km)

CORE The core is SYNCHRONIZED ORBITS made of iron, nickel, and The orbital arrangement of Pluto and Charon is unique. Each an observer on one side of Pluto would be able to see Charon, silicates. always faces the other, making the two seem connected by an but another observer standing on the other side of the planet invisible bar. The synchronization of the two bodies is such that could not see this moon due to the curvature of the planet.

MANTLE The mantle is a layer of frozen water. 570 miles (920 km)

BEST VIEW OF PLUTO AVAILABLE

NEW HORIZONS MISSION DENSITY The first space probe to be sent Moons to Pluto was launched on January 19, Charon's density is between 2006. It is to reach the dwarf planet 0.7 and 0.8 ounce per cubic In addition to Charon, which was in July 2015 and achieve the first inch (1.2 and 1.3 g/cu cm), discovered in 1978, Pluto is orbited by flyby of Pluto and Charon. indicating that its composition two additional moons, Nix and Hydra, first does not include much rock. observed in 2005. Unlike the surface of Pluto, which is made of frozen nitrogen, methane, and carbon dioxide, Charon appears to be covered with ice, methane, and carbon A PECULIAR ORBIT dioxide. One theory holds that the matter that Pluto's orbit is noticeably elliptical, and it is tilted 17° formed this satellite was ejected from Pluto 730 miles from the plane of the planets' orbits. The distance 6,387 as a result of a collision, an origin similar to between Pluto and the Sun varies from 2,500,000,000 that ascribed to Earth's moon. to 4,300,000,000 miles (4,000,000,000 to terrestrial days is the time (1,172 km) 7,000,000,000 km). During each 248-year orbit, Pluto Pluto takes to complete Charon's diameterΩhalf one rotation. of Pluto's orbits closer to the Sun than Neptune for nearly 20 years. Although Pluto appears to cross paths with Neptune, it is impossible for them to collide. 60 THE SOLAR SYSTEM UNIVERSE 61

Comparable Sizes Distant Worlds The discovery of Quaoar in 2002 allowed scientists to find the link arther even than Neptune, the eighth planet, we find frozen they had long looked for between the Kuiper belt and the origin of the solar bodies smaller than the Earth's Moon—the more than 100,000 system. Quaoar's almost circular orbit F objects forming the Kuiper belt, the frozen boundary of our solar system. helped prove that some objects both belong to the Kuiper belt and orbit the Sun. At the Recently astronomers of the International Astronomical Union decided to reclassify Pluto as official meeting of the International a dwarf planet because of its size and eccentric orbit. Periodic comets (comets that appear Astronomical Union, on August 24, 2006, Pluto was reclassified from a planet to a QUAOAR SEDNA PLUTO ERIS at regular intervals) originate in the Kuiper belt. Nonperiodic comets, on the other hand, dwarf planet. For the time being, any has a diameter Its diameter is possesses a Larger than Pluto, its come from the Oort cloud, a gigantic sphere surrounding the entire solar system. further objects discovered in the Kuiper belt of 810 miles estimated at 1,000 diameter of 1,400 diameter is about 1,900 will be classified in the same category. (1,300 km). miles (1,600 km). miles (2,300 km). miles (3,000 km).

OR MORE, POSSIBLE EXTRASOLAR PLANETS 200 HAVE BEEN DETECTED.

SATURN'S URANUS'S NEPTUNE'S ORBIT ORBIT ORBIT

PLUTO'S ORBIT

ERIS

THE FARTHEST ONE

Eris is 97 astronomical units Kuiper Belt 1,410 miles (9,040,000,000 miles [14,550,000,000 km]) from the Sun, making it the most distant object observed in the solar Extending outward from the orbit of Neptune are many frozen worlds similar in some ways to system. This dwarf planet follows an planets but much smaller. They are located in the Kuiper belt, the frozen boundary of our solar (2,274 km) oval, eccentric orbit that takes 560 years system. So far, almost a thousand objects have been cataloged, including Quaoar, which has a is the diameter of Pluto—750 miles (1,200 to complete. The dwarf planet measures diameter of 810 miles (1,300 km). The Kuiper belt, estimated to contain more than 100,000 bodies of km) smaller than the Earth's Moon. Because about 1,900 miles (3,000 km) in diameter, and traces of methane ice have ice and rock larger than 60 miles (100 km) in diameter (including Pluto), spreads out in the shape of a of its size and orbit, Pluto is considered a been detected on its surface. wide ring. Many of the comets that approach the Sun come from the Kuiper belt. dwarf planet instead of a planet. 62 THE SOLAR SYSTEM UNIVERSE 63

Construction Debris: 15% The percentage of the Asteroids total mass of the asteroids compared with Also called minor planets, they are the millions of rock and the mass of the Moon Asteroids and Meteorites metal fragments of various shapes and sizes that orbit the Sun. They are mostly located in a belt between the orbits of Mars and Jupiter, but a few, such as those that belong to the Amor, ver since the formation of the solar system, the melting, collision, and rupture of Apollo, and Aten asteroid groups, orbit closer to Earth. E various materials played an essential role in the formation of the planets. Remnants of this process remain in the form of rock debris, which serves as witnesses to the formation of the solar system. These objects are also associated with episodes that HIDALGO completes a solar orbit influenced subsequent evolutionary processes on Earth. They are a possible cause of the every 14 Earth years. mass extinction of dinosaurs more than 60 million years ago. ATEN

AMOR Tighten Your Belt APOLLO More than a million asteroids at least a TYPES OF METEORITES mile in diameter are distributed in the Extraterrestrial MAIN ASTEROID BELT main asteroid belt. Ceres was the first asteroid One of the main goals of scientist who study meteorites is discovered (in 1801). It is the largest known asteroid, with a diameter of 580 miles (932 km). to understand their nature. Meteoric material holds The Trojans trace an orbit extraterrestrial solids and gases. Scientific tests have confirmed STONY similar to Jupiter's, one Mars's Jupiter's that some meteorites are from the Moon or Mars, but most group in front of the planet KIRKWOOD meteorites contain Orbit Orbit meteorites are associated with asteroids. The samples obtained silicate minerals. and another behind it. The Kirkwood gaps from meteorites are analyzed and classified by their composition. They are subdivided are the open areas into chondrites and in the main asteroid achondrites. belt that are devoid of asteroids. A HUGE METEORITE STRIKES TYPES OF ASTEROIDS Despite a great variety in size and shape, three types of A meteorite is an object from space that does not completely minor planets, or asteroids, are known. Classified by vaporize as it penetrates the Earth's atmosphere. Larger their composition, they are grouped as siliceous, meteorites can form a crater when they strike the Earth. Shown is carbonaceous, and metallic. the impact of an exceptionally large meteorite, such as the one that many scientists believed might have led to the IRON extinction of dinosaurs and many other species meteorites contain a about 65 million years ago. high percentage of IDA iron and nickel An asteroid 35 miles compounds. They are (56 km) long, its surface created in the rupture is marked by collisions EXPLOSION of asteroids. with other bodies. The friction created as a 1. meteorite falls through the air increases its temperature. This is how an ignition process is started. MESOSIDERITES contain similar quantities of iron, 7 miles per second nickel, and silicates. (12 km/s) IMPACT VELOCITY DIVISION The fragmentation of 2. a meteorite causes a visual effect called a shooting star.

43 miles per second (70 km/s) IMPACT The collision of the 3. meteorite compresses and excavates the ground, leaving a crater.

Ferrous-type rocks dominate its composition. 64 THE SOLAR SYSTEM UNIVERSE 65 Those with a Tail THE HEAD The head of a comet omets are small, deformed objects a few miles in diameter that are can measure 62,000 miles (100,000 km) or normally frozen and dark. Made of dust, rock, gases, and organic more in diameter. C molecules rich in carbon, comets are usually found in orbits beyond that of Neptune in the Kuiper belt or in the Oort cloud. Occasionally a comet, such as Halley's comet, veers toward the interior of the solar system, where its ice is heated and sublimates, forming a head and long, spectacular tails of gases and dust.

LONG-PERIOD SHORT- ION TAIL Types of Comets COMET PERIOD Periodic comets The trail of suspended Comets with short periods COMET gases generates a low- Comets that leave their original intensity, luminous region have orbits around the Sun orbits and approach the Sun with a bluish color. The gas that are shorter than 200 years. generally settle into new trajectories. molecules lose an electron Those with a period of more than KUIPER Halley's comet, for example, completes and therefore have an electrical charge. 200 years travel tens or hundreds BELT its elongated orbit in 76 years. of times farther from the Sun than Pluto. OORT SOLAR HEAD CLOUD SYSTEM Formed by the nucleus and the coma. The front part is called the impact front. COMA Envelops the nucleus. Deep Impact Mission It is formed by the gases and dust that On January 12, 2005, as part of the PROBE LAUNCH it releases. DUST TAIL The suspended dust Discovery program, NASA launched Deep Impact launches the the space probe Deep Impact, which, in particles trail behind the 1. 770-pound (350-kg) comet, reflecting sunlight turn, sent a projectile on a collision course copper projectile that will and making the luminous toward the comet 9P/Tempel 1, where it collide with the comet. tail visible. obtained samples to be studied on Earth. NUCLEUS Frozen water, methane, carbon dioxide, ammonia, rock, and dust

The projectile searches the impact front. IN POSITION By means of 2. infrared cameras and spectrometers, SOLAR the ship follows the WIND comet to analyze the impact on the nucleus.

NUCLEUS

22,370 IMPACT WITH THE COMET took place on July 4, 2005. Close to the Sun, As the comet miles per hour The projectile generated a the tails reach moves away from 3. the Sun, its tails crater the size of a football FORMATION OF THE TAIL maximum length. (36,000 km/h) disappear. field and seven stories deep. AND HEAD VELOCITY OF THE COMET IMPACT Because of the effects of solar radiation and the solar wind, gases and dust are released from an accelerating comet. The dust Sun Earth Mars PREVIOUS MISSIONS particles tend to form a curving trail, which GIOTTO DEEP SPACE STARDUST is less sensitive to the pressure of the solar NASA has sent other unmanned missions to Launched in 1986, The NASA spacecraft In 2004, it took wind. As the comet leaves the confines of approached the comets. The first was the international craft it passed the nucleus samples of the the solar system, its tails coincide once ISEE-3/ICE. Launched in 1978, its mission of Halley's comet at a comet Borrelly comet Wild 2 ended after crossing the tail of the Giacobini- distance of 310 miles in 2001. and sent them more, but they disappear as the nucleus Jupiter Comet orbit Zinner comet in September 1985. (500 km) in 1992. to Earth. cools down and ceases releasing gases. AERIAL VIEW OF THE EARTH THE BLUE PLANET 68-69 THE MOON AND TIDES 76-77 In this partial image of the Earth, we can see Bora-Bora, an island that forms part of the JOURNEY TO THE CENTER OF THE EARTH 70-71 ECLIPSES 78-79 Leeward Islands, located in French Polynesia. The Earth and the Moon ONCE UPON A TIME 72-73 MOVEMENTS AND COORDINATES 74-75

n the beginning, the Earth was an many drastic changes took place during atmosphere—the colorless, odorless, approximately 435 miles (700 km) thick, incandescent mass that slowly these early eras, our blue planet has still invisible layer of gases that surrounds it has no clear boundary and fades into began to cool, allowing the not stopped changing. It must be us, giving us air to breathe and space until it finally disappears. I continents to emerge and acquire recognized that life on Earth would be protecting us from the Sun's harmful their current form. Although impossible without the presence of the radiation. Although the atmosphere is 68 THE EARTH AND THE MOON UNIVERSE 69

AXIS CHARACTERISTICS Density 3.2 ounces per cubic INCLINATION inch (5.52 g/cu cm) CONVENTIONAL Average The Blue Planet PLANET 23.5° temperature 59° F (15° C) SYMBOL ROTATION This is the inclination of the *In both cases, Earth = 1 he Earth is known as the blue planet because of the color Earth’s axis from the AXIS vertical. As the Earth orbits ESSENTIAL DATA of the oceans that cover two thirds of its surface. This planet, the Sun, different regions AXIS INCLINATION T gradually receive more or Average distance 93 million miles the third planet from the Sun, is the only one where the right NORTH to the Sun less sunlight, causing the (150 million km) 23.5° conditions exist to sustain life, something that makes the Earth POLE four seasons. Revolution around the Sun (Earth year) 365.25 days One rotation special. It has liquid water in abundance, a mild temperature, and Diameter at 7,930 miles lasts 23.56 an atmosphere that protects it from objects that fall from the equator (12,756 km) hours. Orbiting 17 miles per second outer space. The atmosphere also filters solar radiation speed (27.79 km/s.) thanks to its ozone layer. Slightly flattened at its poles Mass* 1 PRECIPITATION Gravity* 1 and wider at its equator, the Earth takes 24 hours to 3. The atmosphere loses water through revolve once on its axis. condensation. Gravity causes rain, snow, and Magnetism and Gravity hail. Dew and frost The Earth’s magnetic field originates in the planet’s outer core, directly alter the state of where turbulent currents of molten iron generate both electric and the surface they cover. magnetic fields. The orientation of the Earth’s magnetism varies over time, causing the magnetic poles to fluctuate. The Phenomenon of Life 70% Magnetic Water, in liquid form, makes it possible for life to The Earth’s force exist on the Earth, the only planet where of the Earth’s surface core works as temperatures vary from 32° F to 212° F (0° C to 100° C), is water. From space, a magnet. the planet looks blue. allowing water to exist as a liquid. The Earth’s average distance from the Sun, along with certain other factors, Solid core allowed life to develop 3.8 billion years ago.

Mantle

CONDENSATION The Earth’s winds transport The Earth’s magnetic 2. field is created by moisture-laden air until The liquid convective currents weather conditions cause the outer core is in its outer core. water vapor to condense into in constant clouds and eventually fall to the motion. ground as rain or other forms of precipitation. -76° F 32° to 212° F Above 212° F WHAT IT DOES (-60° C) Some particles are (0° to 100° C) (100° C) attracted to the poles. The magnetic field ONLY ICE 3 STATES ONLY STEAM Van Allen belt Mars is so far from On the Earth, water is On Mercury or Venus, which protects the Earth the Sun that all its found in all three of its are very close to the Sun, from the radiation water is frozen. possible states. water would evaporate. of the solar wind. EVAPORATION 1. Because of the Sun’s energy, the water Solar wind Magnetic evaporates, entering the field lines EARTH MOVEMENTS atmosphere from oceans and, to a lesser extent, Magnetosphere The Earth moves, from lakes, rivers, and other sources on the orbiting the Sun and continents. rotating on its own axis. The Van Allen belts trap the particles from the solar Axis wind, causing phenomena Magnetic field tail like the auroras. Earth

SUN 93,500,000 miles (149,503,000 km)

pounds pounds pounds GRAVITY AND 24 (11 kg) 154 (70 kg) 390 (177 kg) WEIGHT The Moon, our only natural SOUTH ON THE MOON ON EARTH ON JÚPITER ROTATION: The Earth REVOLUTION: It takes the Earth 365 satellite, is four times smaller POLE Weight is the force of The Moon has less mass The object is drawn Jupiter has 300 times more revolves on its axis in 23 days, 5 hours, and 57 minutes to travel than the Earth and takes the gravity that acts than the Earth and, as a toward the Earth’s center. mass than the Earth and hours and 56 minutes. once around the Sun. 27.32 days to orbit the Earth. on a body. result, less gravity. therefore more gravity. 70 THE EARTH AND THE MOON UNIVERSE 71 Journey to the Center EXOSPHERE

WITHOUT of the Earth ATMOSPHERE WITH ATMOSPHERE Direct solar radiation. e live on the Earth, but do we know what we are The sunlight filters into Differences in the atmosphere. Winds temperature between standing on? The planet is made up of layers of THERMOSPHERE distribute the heat, the equator and the

435 miles cooling the tropics and poles would be far W (700 km) various materials, such as solid and molten warming the poles. more pronounced. rock, which in turn are composed of such elements as iron, nickel, and silicon. Our atmosphere is the Above the Surface layer of gases surrounding our planet. One MESOSPHERE The existence of life on our planet would be of these gases, oxygen, does a very special impossible without the atmosphere that provides job—it permits life to exist. the air we breathe and the water we drink; it also 1,800 miles (2,900 km) protects us from the Sun’s harmful radiation, while simultaneously maintaining mild temperatures by Internal Structure STRATOSPHERE retaining the Sun’s warmth. The atmosphere is about 435 miles thick (700 km) but lacks defined limits. We live on a rocky surface similar to a shell. The rocks we live with are made mostly of oxygen and silicon, but underneath them is the mantle, made of much heavier rocks. The mantle also surrounds the inner and 620 miles outer cores with a region of constantly boiling liquid metals, TROPOSPHERE (1,000 km) It fades into outer space. creating the convective currents that generate the Earth’s magnetic field. The inner core, solid because of the great pressure put upon it, is the densest part of the planet. OUTER MANTLE Hydrosphere and 370 miles As a result of the high (600 km) temperatures, the materials Lithosphere dilate and produce a The lithosphere includes the crust and the upper portion HOW FAR HUMANS HAVE GONE 1,410 miles (2,270 km) continuous ascending Orbit of an artificial movement that generates of the mantle, and the satellite Mount Everest Continental Oceanic INNER MANTLE convection currents and the hydrosphere includes liquid The solid, intermediate 5.5 miles penetration penetration forces that cause the water, covering 71 percent of layer between the core (8.85 km) changes to the Earth’s crust. the Earth’s surface in lakes, 50 miles and the crust. High- rivers, and five oceans. (80 km) temperature S and P Air is very rarefied. waves pass through it because of its contact with the core. 30 miles The ozone layer, located (50 km) OUTER CORE here, absorbs the Sun’s The outer layer of the core is ultraviolet rays. 1 mile liquid, consisting of molten iron and nickel. Its temperature is (1.7 km) 755 miles (1,216 km) 7 miles lower than that of the inner (11 km) 7.5 miles core and it is under less Vegetable and animal (12 km) pressure. The motion of the life. molten material produces the 0 miles geomagnetic field. (0 km) INNER CORE is made of the same metals as the outer core, but, despite its high temperature, its center is solid because of the enormous pressure that Lithosphere and Hydrosphere compresses it. The hydrosphere, the liquid part of the Earth, 71 percent of the Earth’s surface. The lithosphere includes the oceans, lakes, rivers, underground is a superficial, elastic region that is 4 to 7 miles waters, snow, and ice. It almost completely covers the (6 to 11 km) thick under the oceans and up to 43 crust, surrounds the shores of the continents, and covers miles (70 km) thick under mountain ranges.

WATER AND EARTH TOTAL VOLUME OF WATER FRESHWATER

1.7 % 4.3 % ice 620 MILES 3,965 miles 29.2% 6 % (1,000 KM) 70.8% 94 % under- (6,380 km) soil water salt fresh ground water water from the Earth’s surface to 0.03% its center. surface and 3,965 MILES atmosphere (6,380 KM) 72 THE EARTH AND THE MOON UNIVERSE 73

Origin of the Earth Once Upon a Time The Earth was formed 4.6 billion years ago from a cloud of dust and gas. In the beginning, it was a he Earth probably formed from material in the solar nebula—the cloud of gas and dust molten, constantly active, mass. As time passed, the Earth began to cool, and the atmosphere began to clear as rain that led to the formation of the Sun. This material gradually grew into a larger and larger fell, creating the oceans. T body that became a red-hot ball of rock and metal. Later the rocky crust formed, its surface cooling enough to allow the continents to appear. Even later the oceans arrived, as well A Ball of fire B The planet cooled C The crust forms D Water appeared on the as the tiny organisms that released oxygen into the atmosphere. Although much of this gas was The Earth was The atmosphere Lava poured across Earth 3.9 billion years created from small was created as the the surface of the ago. Water-rich Earth is initially consumed in chemical reactions, over time, it allowed the development of multicellular EURASIA particles that planet cooled and Earth. As it cooled, the only planet in the coalesced in the began to emit it formed the solar system known to organisms and an explosion of life that took place at the start of the Paleozoic solar nebula. gases and steam. Earth’s crust. have life. Era, 542 million years ago.

INDIA Chronology LAURASIA AFRICA Geology is the study of rocks in the Earth’s Through the study of fossils—remains of creatures crust. It divides the Earth’s history into buried in the Earth’s various sedimentary layers Continental Drift different eras, periods, and epochs lasting millions and consequently at different times in the past– We live on the continents, which are part of movable plates that of years. Geology also helps us catalog the geology helps us trace the timeline of evolutionary drift across the Earth’s surface at the rate a fingernail grows. 250 AMERICA processes of evolution—changes in generations as history. million years ago, India, Africa, Australia, and Antarctica were part species adapt to their environment and their competitors. of the same continent. When tectonic plates rub against each AFRICA AUSTRALIA other, land and oceanic crust earthquakes occur. Where the INDIA plates separate, a rift forms. The mid-ocean ridges that SOUTH HOMO LAURASIA AMERICA SAPIENS run beneath the oceans are formed by lava that ANTARCTICA emerges from the rifts between tectonic plates. Where plates collide, a process called subduction takes place, in which the rocks of the oceanic floor are Tethys Sea 60 MILLION YEARS AGO drawn under the continent and melt, reemerging in LARGE The northern Atlantic Ocean slowly MAMMAL the form of volcanoes. 4 separated, completing the formation of SMALL Europe and North Africa. ANTARCTICA MAMMAL

Panthalassa GONDWANA 163 MILLION YEARS AGO Gondwana split, forming Africa and South 3 America as the southern Atlantic Ocean was created. DINOSAUR

PANGEA MARINE REPTILE 250 MILLION YEARS AGO The Tethys Sea slowly split Pangea, 2 creating two continents, known as Laurasia and Gondwana. CONIFERS

ICYTHYOSTEGA 290 MILLION YEARS AGO (amphibian) The supercontinent called Pangea 1 formed. An immense ocean called Panthalassa surrounded it. COOKSONIA (plant)

CRINOID CENOZOIC ERA (sea animal) FOSSILS TRILOBITE are remains of living beings preserved in the rocks as a record of the Earth’s history. MESOZOIC ERA

UNICELLULAR ORGANISM Water The majority Soft are marine Tectonic Plates sediment shells. PALEOZOIC ERA The surface of the Earth is shaped by tectonic Hard THE EARTH IN ONE DAY plates. There are eight major plates, some of sediment At a certain If the Earth’s history were compressed into which even encompass entire continents. The plates’ depth, the pressure a normal day, Homo sapiens would appear borders are marked by ocean trenches, cliffs, chains Metamorphic destroys at just one minute to midnight. of volcanoes, and earthquake zones. rocks the fossils.

PRECAMBRIC TIME 74 THE EARTH AND THE MOON UNIVERSE 75

December 21 or 22 APHELION Movements and Coordinates 23.5º The point in the TILT OF THE Winter solstice in the Northern Earth’s orbit where it es, it moves. The Earth rotates on its axis while simultaneously orbiting the Sun. The EARTH’S AXIS Hemisphere and summer solstice in is farthest from the Sun (94 million miles natural phenomena of night and day, seasons, and years are caused by these movements. the Southern Hemisphere. Solstices exist because of the tilt of the [152 million km]). This Y To track the passage of time, calendars, clocks, and time zones were invented. Time occurs at the Earth’s axis. The length of the day and the beginning of July. zones are divided by meridians and assigned a reference hour according to their location. When height of the Sun in the sky are greatest in traveling east, an hour is added with each time zone. An hour is subtracted summer and least in winter. during west-bound travel. 93 Geographic Coordinates Thanks to the grid formed by the lines of latitude and longitude, the Equinox and Solstice MILLION MILES position of any object on the Earth’s surface can be easily located by The Earth’s Movements (149 MILLION KM) using the intersection of the Earth’s equator and the Greenwich meridian Every year, around June 21, the Northern Hemisphere reaches (longitude 0°) as a reference point. This intersection marks the midpoint Night and day, summer and winter, new year and its maximum inclination toward the Sun (a phenomenon between the Earth’s poles. old year result from the Earth’s various movements referred to as the summer solstice in the Northern Hemisphere and the winter solstice in the Southern Hemisphere). The North 0° during its orbit of the Sun. The most important motions GREENWICH Pole receives sunlight all day, while the South Pole is covered in are the Earth’s daily rotation from west to east on its own MERIDIAN darkness. Between one solstice and another the equinoxes axis and its revolution around the Sun. (The Earth follows THE EARTH’S appear, which is when the axis of the Earth points toward the Northern ORBIT Hemisphere an elliptical orbit that has the Sun at one of the foci of the Sun and the periods of daylight and darkness are the same all About 365 days PARALLELS ellipse, so the distance to the Sun varies slightly over the over our planet. course of a year.) 66.5° N Arctic Circle Temperate 23.5° N Tropic of Cancer N zone ROTATION June 20 or 21 March 1 day 23º 1 DAY THE DAYS The Earth revolves Summer solstice in the Northern Hemisphere 20 or 21 Period of time it takes once on its axis in 23 and winter solstice in the Southern Hemisphere. the Earth to rotate on Spring equinox EQUATOR hours and 56 minutes. Solstices exist because of the tilt of the Earth’s axis. its axis 0 ° in the Northern SUN We see this as day The length of the day and the height of the Sun in the Tropical and night. Hemisphere zone 23.5° S Tropic of Capricorn sky are greatest in summer and least in winter. Antarctic Circle S and autumn Polar 66.5° S About 30 days THE MONTHS equinox in the zone Each period of time, Southern Southern between 28 and 31 Hemisphere Hemisphere. days, into which a year The Sun passes is divided REVOLUTION directly above the 1 YEAR equator, and day The Earth’s orbit and night have the Time Zones 12:00 around the Sun lasts same length. A.M. 365 days, 5 hours, The Earth is divided into 24 areas, or time England, meridian as the base meridian. One and 57 minutes. zones, each one of which corresponds to hour is added when crossing the meridian in WEST EAST an hour assigned according to the Coordinated an easterly direction, and one hour is Universal Time (UTC), using the Greenwich, subtracted when traveling west. 3:00 A.M. 9:00 P.M. JET LAG The human body’s biological clock responds to the rhythms of light and dark based on the passage of night and day. Long air flights east or west interrupt and disorient the body’s 3º NUTATION 18.6 YEARS clock, causing a disorder known as jet lag. It can cause fatigue, irritability, nausea, is a sort of nod made September headaches, and difficulty sleeping at night. by the Earth, causing 21 or 22 the displacement of 6:00 A.M.N 6:00 P.M. the geographic poles Autumn equinox in the 12:00 A.M. Northern 12:00 P.M. Hemisphere by nine arc seconds. Northern Hemisphere Departure Arrival time and spring equinox in the time Southern Hemisphere. PRECESSION The Sun passes directly above 25,800 YEARS the equator, and day and 47º A slow turning of the night have the same length. 9:00 A.M. direction of the Earth’s 3:00 P.M. axis (similar to that of MEASUREMENT OF TIME a top), caused by the Months and days are charted by calendars and clocks, PERIHELION Earth’s nonspherical but the measurement of these units of time is neither a The point where the orbiting Earth shape and the cultural nor an arbitrary construct. Instead, it is most closely approaches the Sun 12:00 gravitational forces of derived from the natural movements of the Earth. (91 million miles [147 million km]) 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 P.M. the Sun and the Moon 76 THE EARTH AND THE MOON UNIVERSE 77

CHARACTERISTICS ORIGIN OF THE MOON INNER STRUCTURE The most widely accepted Various seismic analyses of the Moon suggest that its CONVENTIONAL theory of the Moon’s origin The Moon and Tides core is solid or semisolid. PLANET suggests that an object the size SYMBOL of Mars collided with the Earth during its formation. miles omance and terror, mystery and superstition–all these ESSENTIAL DATA (3,476 km) Mare Crisium emotions are responses to the Moon, the Earth’s one natural 2,160 Measures 280 Average distance 226,400 miles The diameter of the Moon is one miles by 370 from the Earth (364,400 km) The ejected material ROCKY 1000 km miles (450 km R fourth of the Earth’s. Revolution satellite, which always hides one of its two faces. However, scattered into space MANTLE by 563 km) around the Earth around the Earth, and Less than half and has large 27.3 days whatever symbolic meanings are attributed to the Moon, its the thickness over time, it coalesced craters. Diameter at 2,160 miles of the Earth’s gravitational pull has a concrete effect on the Earth—it is a cause of into the Moon. the equator (3,476 km) Mare Imbrium mantle Mare is 3.85 billion Tranquillitatis Orbiting speed 0.6 miles per the tides. Depending on the distance of the Moon from the Earth, the years old. The seas are second (1.02 km/s) OUTER flatlands with few gravitational pull exerted by the Moon varies in strength and so can Mass* CORE craters. 0,01 high tides and low tides. To reach full height, tides need large open Partially Gravity* 0,17 melted Density 2 ounces per cubic inch (3.34 g/cu cm)

areas of ocean. For this reason, tides in closed or small bodies of 100 km Temperature 302° F (150° C) (day) water are much lower. -148° F (-100° C) (night) Aristarchus Brightest spot Volume* 0.02 on the Moon *Earth = 1 THE MOON’S MOVEMENTS INNER Oceanus AXIS INCLINATION CORE As the Moon orbits the Procellarum Central LUNAR MONTH SIDEREAL MONTH The largest sea, Earth, it revolves on its own It takes 29.53 It takes 27.32 days temperature 5.14° it is not well axis in such a way that it days to complete to orbit the Earth. HIDDEN of preserved. 2,730° F One rotation always shows the Earth the its phases. FACE (1,500° C) lasts 27.32 same side. Invisible from the Earth, this Earth days. side of the Moon was a mystery until 1959, when the Russian probe VISIBLE FACE CRUST Surface made of rocks, such as granite, covered Visible face Moon Spotted with dark Luna 3 managed to photograph the by 65 feet (20 m) of lunar dust called regolith hidden zone. Because of the areas, it always Hidden Earth faces the Earth. Mare face greater thickness of the Moon’s Nubium crust on this side, it has The Lunar Landscape fewer seas. Observing the Moon, the ancient astronomers Lunar orbit Grimaldi decided that, as on the Earth, its plainly visible dark spots must be seas. These dark regions of the Moon contrast against the bright ones, the highlands with the Gassendi most impact craters. 1 NEW MOON Mare The Tides SPRING TIDE Morum When the Sun and the Moon Rupes Altai Humboldt The water on the side of the Earth are aligned, the highest high Mountain chain Crater named in closest to the Moon feels the Moon’s tides and lowest low tides 5,900 feet honor of the MOUNTAIN gravitational pull most intensely, and vice are produced. FIRST QUARTER (1,800 m) high German naturalist RANGES 2 NEAP TIDE versa. Two tides are formed, and they track The Moon and the Sun are at When a meteorite strikes the Moon in its orbit around the Earth. right angles to the Earth, the lunar surface, a However, they precede the Moon instead of producing the lowest high tides mountain range forms being directly in line with it. and the highest low tides. Montes Apenninus from the material ejected One of the most during the cratering notable mountain ranges process. FULL MOON 3 SPRING TIDE The Sun and the Moon align once again, and the Sun augments the Moon’s gravitational pull, causing CRATERS SEAS Lunar orbit KEY a second spring tide. can be from 40 inches (1 m) cover almost 16 percent of to 620 miles (1,000 km) in the Moon’s surface and Gravitational Moon Schickard diameter and are formed by were formed by flowing pull of the Moon Earth Tycho Maguinus 100 million orbit Copernicus meteorites that strike the lava. Today the Moon has years old 60 miles Moon’s surface with no volcanic activity. THIRD QUARTER (93 km) in Gravitational 4 NEAP TIDE diameter incredible force. pull of the Sun The Moon and the Sun again form a right angle, causing a Influence on second neap tide. the tide by the THE PHASES OF THE MOON gravitational pull of the Sun 46,6 % Influence on the tide by the The Sun’s gravity Unique also influences gravitational Sun The Moon is the Earth’s pull of the the tides. New Waxing First Waxing Full Waning Third Waning only natural satellite. Moon Moon crescent quarter gibbous Moon gibbous quarter crescent 78 THE EARTH AND THE MOON UNIVERSE 79

Lunar Eclipse Eclipses When the Earth passes directly between the ALIGNMENT TYPES OF ECLIPSES full Moon and the Sun, a lunar eclipse (which could be total, partial, or penumbral) occurs. Sun Earth Moon ypically four times a year, during the full or new Without the Earth’s atmosphere, during each lunar moon, the centers of the Moon, the Sun, and the TOTAL LUNAR eclipse, the Moon would become completely invisible ECLIPSE, SEEN (something that never happens). The totally eclipsed T FROM THE EARTH Earth become aligned, causing one of the most Moon’s characteristic reddish color is caused by light TOTAL PARTIAL PENUMBRAL marvelous celestial phenomena: an eclipse. At these times, The orange color comes from refracted by the Earth’s atmosphere. During a partial The Moon is The Moon is The Moon is in sunlight that has been refracted eclipse, on the other hand, part of the Moon falls in completely in only partially the penumbral the Moon either passes in front of the Sun or passes through and colored by the Earth’s the shadow cone, while the rest is in the penumbra, the shadow inside the cone. atmosphere. During an eclipse, the Moon is not completely the outermost, palest part. It is not dangerous to cone. shadow cone. the Earth’s shadow. The Sun—even during an eclipse—is not black but appears reddish. safe to look at directly, since it can cause irreparable damage look at a lunar eclipse directly. to the eyes, such as burns on the retina. Special high- quality filters or indirect viewing by projecting the Sun’s image on a sheet of paper are some of the ways in which FULL this celestial wonder can be watched. Solar eclipses MOON TOTAL provide, in addition, a good opportunity for astronomers to Shadow ECLIPSE cone conduct scientific research. ANNUAL ECLIPSE Lunar OF THE SUN, SEEN orbit FROM THE EARTH

Solar Eclipse PARTIAL ECLIPSE Solar eclipses occur when the Moon ALIGNMENT TYPES OF ECLIPSES passes directly between the Sun and the Earth, casting a shadow along a path on the Sun Moon Earth Earth’s surface. The central cone of the shadow is called the umbra, and the area of partial shadow around it is called the penumbra. Viewers in the regions where the umbra falls on the Earth’s surface see the Moon’s disk completely obscure the Sun—a total solar TOTAL ANNULAR PARTIAL eclipse. Those watching from the surrounding During a solar eclipse, astronomers take The Moon is The Sun appears The Moon does PENUMBRAL Shadow areas that are located in the penumbra see the advantage of the blocked view of the Sun in between the Sun larger than the not cover the Sun ECLIPSE Moon’s disk cover only part of the Sun—a order to use devices designed to study the and the Earth and Moon, and it completely, so the cone Sun’s atmosphere. partial solar eclipse. creates a cone- remains visible Sun appears as a Penumbra shaped shadow. around it. crescent. NEW MOON cone TOTAL EARTH ECLIPSE

Earth orbit

THE ECLIPSE CYCLE OBSERVATION FROM EARTH SOLAR ECLIPSES LUNAR ECLIPSES Eclipses repeat every 223 lunations—18 years and 11 days. are different for each are the same for all SUNLIGHT These are called Saros periods. A black, polymer filter, with Prevents local observer. observers. an optical density of 5.0, retinal burns ECLIPSES IN A YEAR ECLIPSES IN A SAROS produces a clear orange image of the Sun. 2 7 4 41 29 70 MAXIMUM DURATION MAXIMUM DURATION Minimum Maximum Average of the of the Total Sun Moon 8 minutes 100 minutes ECLIPSES IN 2006 AND BEYOND

OF THE 3/29 9/22 3/19 9/11 2/07 1/26 7/22 1/15 7/11 1/4 11/25 5/20 11/13 5/10 11/3 4/29 10/23 3/20 9/13 DISTANCE FROM THE SUN TO SUN Total Total Partial Partial Total Total Total Total Total Partial Partial Annular Annular Annular Hybrid AnnularPartial Total Partial SUN’S APPARENT SIZE THE EARTH times larger times greater than 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 than the the distance from the OF THE 3/14 9/07 3/03 8/28 2/21 8/16 2/9 7/7 6/26 12/21 6/15 12/10 6/4 12/28 4/25 10/18 4/15 10/08 4/4 9/28 400 Moon 400 Earth to the Moon MOON Partial Partial Total Total Total Partial Partial Partial Partial Total Total Total Partial Partial Partial Partial Total Total Total Total STONEHENGE ASTRONOMICAL THEORIES 82-83 Located in Wiltshire (England), it was built in several phases over some 600 years—between 2200 and SPRINKLED WITH STARS 84-85 1600 BC. The placement of most of its large stones Observing the Universe has a relationship to the Moon and the Sun. CELESTIAL CARTOGRAPHY 86-87 FROM THE HOME GARDEN 88-89 A FOUR-EYED GIANT 90-91

stronomy was born out of was mixed with superstition and ritual. installed in various locations around the part of an attempt to find planets humankind's need to measure The megalithic monument Stonehenge, planet, we have discovered many new beyond the solar system, because time and seasons, marking the found in southern England, is an example things about the universe. The VLT (Very many astronomers suspect that life is A best times to plant. In ancient of this. Today, thanks to advances in new Large Telescope), astronomy's new not exclusive to the Earth. times, the study of the stars technologies, such as the giant telescopes monster telescope located in Chile, is 82 OBSERVING THE UNIVERSE UNIVERSE 83

Heliocentric Model Astronomical Theories In 1543, a few months before his death, argued that spheres moved in endless, circular orbits. Since Nicolaus Copernicus published the book De the universe and all the celestial bodies were thought to be or a long time, it was believed that the Earth was revolutionibus orbium coelestium, inaugurating spherical, he argued that their movements must also be what is now known as the Copernican circular and uniform (the Ptolemaic system considered the stationary. The Sun, the Moon, and the planets were Revolution. The Polish astronomer planets’ circuits to be irregular). Copernicus reasoned that, F thought to orbit it. To study the sky and calculate developed the heliocentric theory since the movements of the planets appeared to be (from helios, the Greek word for “the irregular, the Earth must not be the center of the universe. its movements, people began to build instruments, such Sun”), which contradicted the These discoveries were contrary to the views promulgated as the astrolabe, armillary sphere, and telescope. The geocentric theory. Copernicus’s by the Roman Catholic Church. In fact, both new postulate inverted the Roman Catholics and telescope revolutionized the conception of the traditional relationship of Protestants suppressed universe. Instead of the Earth being at the center the Sun and the Earth, any writings identifying the Sun as advocating these beliefs. of the universe, it was suggested that the Earth the center of the When Galileo Galilei was and other planets travel around the Sun. The universe and the brought to trial by the Roman Earth as one of Catholic Church for Roman Catholic Church opposed the idea and, many solar advocating the Copernican for a time, persecuted dissident astronomers satellites. theory, he was forced to and banned their theories. Copernicus renounce his views.

Geocentric Model Before telescopes, particular, those of Aristotle, who GALILEO’S TELESCOPE binoculars, and modern had proposed the Earth as the The telescope is thought to have been invented in observatories existed, little was center of the universe, with the 1609 by the Dutch optician Hans Lippershey but known about the Earth. Many celestial objects revolving around had no real scientific application until Galileo believed that the Earth was fixed it). Although other ancient Galilei improved and adapted it to observe and that the Sun, the Moon, and astronomers, such as Aristarchus celestial bodies. Galileo’s first telescope, made of a the five known planets orbited it in of Samus, proposed that the Earth leather tube covered by a lens at each end (one circles. This geocentric model was was round and rotated around the lens convex and the other concave), magnified promoted by the Egyptian Sun, Aristotle’s ideas were objects up to 30 times. Using the telescope, Galileo astronomer Claudius Ptolemy, who accepted as true for 16 centuries, discovered that the Sun’s surface had imperfections in the 2nd century AD compiled and at times Aristotle’s ideas were (sunspots), that the Moon had mountains and the astronomical ideas of the defended and preserved by the craters, and that there were four moons, or ancient Greek astronomers (in Roman Catholic Church. satellites, that traveled around Jupiter.

MEASUREMENTS Noticing that the Sun, the Moon, and the stars moved in cycles, ancient civilizations found they THE TRAVELERS could use the sky as both a clock and a calendar. After many years and great advancements in However, ancient astronomers had technology, scientists decided that space difficulties performing the complex observation conducted only from the Earth’s calculations needed to predict the surface was insufficient. In 1959, the first space positions of stars accurately probe was launched, an automatic vehicle that enough to create a truly precise flew to the Moon and photographed its hidden TIME face. The space probes Voyager 1 and 2 calendar. A useful tool developed This to perform this task was the astrolabe was explored the planets Jupiter, Saturn, Uranus, astrolabe. Its engraved plates used by ancient and Neptune, a milestone in space exploration. reproduce the celestial sphere in Persians. To them, In 2005, Voyager 1 reached the region called two dimensions, allowing the astronomy Termination Shock, the frontier of the solar functioned as a kind elevations of the celestial bodies of agricultural system, representing the farthest region explored to be measured. calendar. by humanity. Both probes carried with them golden discs, named Sounds of Earth, containing sounds and images portraying the diversity of life on Earth.

COSMIC CHARACTERS

2nd Century 16th Century 17th Century 17th Century 17th Century 20th Century Claudius Ptolemy Nicolaus Copernicus Johannes Kepler Galileo Galilei Isaac Newton Edwin Hubble 100-170 1473-1543 1571-1630 1564-1642 1642-1727 1889-1953

Resurrected and compiled the In his De revolutionibus orbium The German astronomer, believer Built the first telescope, a He built upon the ideas of Galileo and In 1929, he began to investigate works of great Greek coelestium, the Polish in Copernicus’s heliocentric model, primitive device with which he developed the theory of universal the expansion of the galaxies, astronomers into two astronomer postulated that the formulated three famous laws of discovered sunspots, four of gravitation, asserting that the allowing scientists to obtain books. His postulates held Sun—not the Earth—was the planetary movement, which Jupiter’s moons, the phases movements of the Earth an idea of the true scale of undisputed authority for center of the universe. This encouraged Galileo to publish of Venus, and craters on the and the celestial bodies are the universe as well as refine centuries. concept is the foundation his research. Moon’s surface. governed by the same the big bang theory. of our own astronomy.. natural laws. 84 OBSERVING THE UNIVERSE UNIVERSE 85

Chi1 Orionis Mythological Characters Different Cultures Sprinkled with Stars Omicron Since ancient times, animal figures have been Orionis In antiquity, each culture recognized certain seen represented in the sky by groups of constellations that other civilizations did not. The onstellations are groups of stars thought to represent Xi Orionis stars. The constellation of Taurus takes its name Chinese see smaller, more detailed patterns in the stars, from its resemblance to a bull. Orion, Cassiopeia, allowing for more precise positional information. different animals, mythological characters, and other figures. Andromeda, and Perseus were named after Various cultures also tend to use varied names for the C characters of Greek tragedies. same constellation. Scorpius is recognized by the people Constellations were invented by ancient civilizations to serve of Mesopotamia, Greece, Rome, Mesoamerica, and as reference points in the Earth’s sky. There are 88 of these Oceania—under different names. collections of stars. Although each star in a constellation appears Mu Orionis related to the others, it is actually very far from them. Not Heka all the constellations are visible at the same time from Pi2 Orionis any one place on the Earth. Betelgeuse SCORPIUS In Greco-Roman mythology, Orion and Scorpius are closely Bellatrix linked. Orion is the giant, Pi3 Orionis handsome, seductive The Sky Changes hunter. ORIGIN The Earth takes one year to orbit the The history of western culture’s Sun. As the planet advances in its orbit, constellations begins with the the nighttime sky changes, allowing different Pi4 Orionis first astronomical observations parts of it to be seen. This is why some made by ancient constellations can only be seen during certain URSA MAJOR Mesopotamian peoples. seasons of the year. In addition, different THE MYSTERY The bear represented by Because we inherited the this constellation is constellations can be seen from different OF GIZA constellations from Greco- The alignment of the three unusual because of its Roman culture, most of the latitudes. Only near the equator is it possible pyramids at Giza in Egypt long tail. The constellations are named after 23.5° to see all 88 constellations. appears to be related to constellations’ shapes figures in classical mythology. the alignment of the three rarely agree perfectly All of the earliest constellations stars of Orion’s belt. with their namesakes. were named by the 16th Pi5 Orionis century. Constellations discovered more recently bear Pi6 Orionis names drawn from science or technology or from exotic Star fauna discovered in various background places across the globe. THE CENTAUR is a creature from Greek Mintaka mythology, half man and half horse. The centaur accompanied Orion Earth Sun Alnilam during his quest to recover his sight. Earth’s Alnitak 88 orbit constellations 13 Babylon OPHIUCHUS The Babylonians conceived of the zodiac 2,000 years ago as a way of measuring Although it is the 13th constellation of time, using it as a symbolic calendar. the zodiac, Ophiuchus is not a part of the zodiac. When astrology began 3,000 years ago, the constellation LIBRA was far from the zodiac. Saiph Rigel was at one point, part of Scorpius. GEMINI ARIES LEO CANCER The stars Castor and has only one very SAGITTARIUS The brightest stars are is the least notable Pollux form the head of bright star, PISCES is located at the center of those of the head and back. of the zodiac’s the twins. TAURUS Hamal, the Arabic Not a very the Milky Way and is full of Regulus is prominent. constellations. is visible even without word for “sheep.” noteworthy nebulae and star groups. binoculars. The brightest star, constellation, it has Aldebaran, is red. no very bright stars. The Constellations of the Zodiac OBSERVING THE CONSTELLATIONS The 13 constellations located within the elliptical plane— Observers in both hemispheres can see the VIRGO constellations of the zodiac. In the Northern through which the Sun passes as seen from Earth—are is a constellation called the constellations of the zodiac. Twelve of these Hemisphere, the southern constellations, such as AQUARIUS CAPRICORN SCORPIUS Scorpius, are difficult to see, and in the Southern with several bright constellations have long formed the foundation of astrology, but Hemisphere, northern constellations, such as has globular clusters is one of the least lies in the direction of stars. Ophiuchus, the 13th, is ignored by astrologers as a new addition. Gemini, are difficult to study. and nebulae visible with prominent the Milky Way and binoculars. constellations. its brightest star is Antares. 86 OBSERVING THE UNIVERSE UNIVERSE 87

HOW TO READ A MAP OF THE SKY Stellar Movements Astronomers divide the celestial sphere into sections, Star The visible regions of the celestial sphere Celestial Cartography allowing them to study the sky in detailed, systematic magnitudes and the ways in which stars move ways. These maps can show a particular region observed through the sky depend upon the observer’s from a certain place at a certain time, or they can s on the Earth, so in heaven. Just as terrestrial maps help us find locations on the Constellations latitude. As an observer moves north or south, merely concentrate on a specific location. To specify the the visible portion of the celestial sphere will surface of the planet, star charts use a similar coordinate system to indicate various position of a point on the surface of the Earth, the change. The elevation of the north or south A geographic coordinates called latitude and longitude are Milky Way celestial bodies and locations. Planispheres, or star wheels, are based on the idea of celestial pole above the horizon determines the used. With the celestial sphere, declination and right apparent motion of the stars in the sky. a celestial sphere (an imaginary globe on ascension are used instead. Observers located which the stars appear to lie and that at the equator see the celestial equator surrounds the planet). Two pass directly over their heads. common types are polar and bimonthly star maps.

AT THE POLES At the North Pole, the stars appear to rotate around the observer’s head. The effect is the same at the South Pole but in the opposite direction.

IN MIDDLE LATITUDES some stars can be seen all year long, but others are only visible during certain months.

E THE CELESTIAL A L SPHERE P O The celestial sphere is imagined to P O extend around the Earth and forms F the basis for modern star H T cartography. The sphere is divided into T AT THE EQUATOR a network of lines and coordinates R H stars can be seen corresponding to those used on the Earth, O E throughout the year, allowing an observer to locate constellations N S rising in the east and on the sphere. The celestial equator is a R T setting in the west. projection of the Earth’s equator, the north and A E L L south celestial poles align with the axis of the Earth, L L and the elliptic coincides with the path along which E A the Sun appears to move. T R S SO E U TH TH OF PO Measuring Distances FULL MOON P LE MA THE SQUARE Once a star or constellation has been BIG DIPPER OF PEGASUS located in the sky, hands and arms can serve as simple measuring tools. A single extended finger, shown in the first Different Types of Charts illustration, can form a one-degree angle Throughout the year, different constellations are visible from the observer’s line of sight and is useful because the Earth moves along its orbit. As the Earth’s for measuring short distances between stars. place in its orbit changes, the night side of the planet faces POLAR The celestial EQUATORIAL The closed palm of the hand forms a 10° different regions of space. To compensate for this shifting sphere is generally Six bimonthly maps depict all 88 angle, and the open hand measures 20°. perspective, there are various kinds of planispheres: north and divided into two constellations, which can be seen south polar maps and bimonthly equatorial maps. polar maps: north over the course of the year. and south. ONE FINGER CLOSED HAND OPEN HAND 88 OBSERVING THE UNIVERSE UNIVERSE 89

SHOOTING STARS MOON Observable Objects Very short flashes of The illuminated face of From the Home Garden light lasting only a the Moon can always be The sky is a very busy place. Not only are there stars fraction of a second seen at some time during and planets, but there are also satellites, airplanes, the night, at least targazing is not difficult. After learning to locate celestial objects, many comets, and meteorites. Fortunately all become recognizable partially, except around by their appearance as well as their movement. people find the hobby very gratifying. With the aid of a star map, you can the new moon. S recognize galaxies, nebulae, star clusters, planets, and other objects. Some of these treasures of the universe are visible with the unaided eye, but others VENUS SATELLITES COMETS require binoculars or even more sophisticated telescopes. Familiarity with the can generally be seen The bigger ones are visible to the naked eye above the horizon at brighter than some stars. appear every one to two night sky is useful in many ways. dusk or dawn. Some take a while to cross years and are visible for the sky. weeks or even months.

Basics BARREL Before stepping out to observe the night sky, make sure you have everything you need. If you collect all your supplies beforehand, you will avoid having to expose your eyes to Flat Perspective bright light once they have adjusted to darkness. In addition to binoculars, star maps, and a A constellation is a group of stars that, when notebook, you should bring warm clothes, a comfortable seat, and something to drink. OBJECTIVE viewed from a certain angle, seem to assume a specific shape. However, these stars that seem closely joined together are, in fact, separated by great distances.

OPTIC TUBE

Planisphere Compass Flashlight with 17,000 red cellophane LIGHT-YEARS FROM EARTH OMEGA TRIPOD ADAPTER How to Look at the Moon Under various degrees of magnification, the Moon and stars take on different appearances. In some cases, you can make observations of the Moon with the unaided eye as well as with binoculars or a telescope. PRISM

FOCUSING WHEEL 4.2 LIGHT-YEARS FROM EARTH Measurement methods CENTAURI A planisphere is a circular star chart that is used to locate celestial bodies in the celestial sphere. To identify Moon 10 TIMES 50 TO 100 FOCUSING a particular object, your own arms and body can be used to LARGER TIMES LARGER EYEPIECE measure its direction and altitude in relation to the horizon.

Normal View with View with view binoculars telescope MEASUREMENT OF ELEVATION MEASURING DIRECTION

ADJUSTMENT 90º 45º 90º 45º 12:00 A.M. SCREW THE MOTION OF CONSTELLATIONS The Earth’s rotation makes the 9:00 P.M. JÚPITER 3:00 A.M. planets and stars appear to move through the nighttime sky in a general east-to-west direction. When the southern Horizon constellation Orion, visible from ORIÓN November through March, is Starting at the horizon, To measure a 45° angle, move The planisphere indicates the A star to the southwest could viewed from the Northern extend one of your arms your arm halfway up from the principal direction of a star. be located with your arms at Hemisphere, it appears to move EAST SOUTH WEST until it is perpendicular to horizon. Place the arms at 90º, using 45°. Combine the directional from left to right. the other. north or south as the base. angles with your hand measurements for elevation. 90 OBSERVING THE UNIVERSE UNIVERSE 91

Telescope units Cerro Paranal Observatory A Four-Eyed Giant MELIPAL The ESO’s Very Large Telescope is located to the north of the Atacama desert, on Cerro Paranal. Completed in he Paranal Observatory, one of the most advanced in the world, is located in the KUEYEN 2006, it has four 26.9-foot- (8.2-m-) wide reflector telescopes capable of observing objects four billion times fainter than those region of Antofagasta, Chile. It uses four identical telescopes to obtain enough light- visible to the unaided eye. It also has three 5.9-foot- (1.8-m-) ANTU YEPUN T wide movable auxiliary telescopes that are used in conjunction gathering power that it could see the flame of a candle on the surface with the larger ones to simulate the light-gathering power of a of the Moon. This sophisticated collection 52-foot- (16-m-) wide mirror (with the resolution of a 656-foot- Light [200-m-] long telescope). This is enough to see an astronaut on of digital cameras, reflecting mirrors, AUXILIARY tunnels for the Moon. The above technique is called interferometry. and other instruments is mounted in TELESCOPE (AT) interferometry DOME There are four, each 5.9 feet the interior of four metallic Its protective cover (1.8 m) in diameter. They Rails to perceives changes assist with interferometry. SQ FT structures weighing hundreds of transport 215,000 in the weather by (20,000 SQ M) TOTAL SURFACE the AT tons. The Very Large Telescope means of thermal (VLT) is operated by a scientific sensors. 7,759 FEET consortium drawn from eight The Telescope (2,365 M) ABOVE SEA LEVEL European countries. One of their The main feature of the VLT is its ADAPTIVE OPTICS stated objectives is to discover revolutionary optical design. By To prevent the primary mirror from deforming because of using adaptive and active optics, it achieves gravitational effects, the VLT has an adaptive optics system new worlds orbiting other stars. resolution similar to that possible from space. that maintains the mirror in optimal shape, with 150 supporting pistons that continually adjust the shape of the mirror. ACTIVE OPTICS CLIMATIC CONDITIONS Cerro Paranal is located in the driest part of the Light Reflected Atacama desert, where the conditions for ADAPTIVE OPTICS enters light beam astronomical observation are extraordinarily 3.9-foot- Uncorrected favorable. It is an 8,645-foot- (2,635-m-) tall (1.2-m-) vision diameter mountain that has about 350 cloudless nights a year. secondary mirror

Mechanical structure 10.9 0.06 150-piston cell Curved Corrected (750) (0.96) mirror vision LB PER SQ IN (MBAR) LB PER CU FT (KG/M3) Air pressure Air density 18–77° (-8–25°) 5 to 20 VLT FAHRENHEIT (CELSIUS) PERCENT ACRONYM FOR Average temperature Humidity Very Large Telescope

ARMILLARY SPHERE Invented by Eratosthenes in the year 225 BC, it was used as a teaching 2500–2000 BC 435–455 BC 1726 1888 1897 1979 aid and became STONEHENGE CARACOL JAIPUR LICK YERKES MAUNA KEA especially popular in Located in Wiltshire, England, it It is located in the ruins of the Located in India, it was built Located on 4,265-foot- (1,300- Located in Wisconsin, it An international complex the Middle Ages is an observatory temple dating Mayan city of Chichén Itzá. The by the maharajah Sawai Jai m-) high Mount Hamilton. It contains the largest located in Hawaii, with large thanks to Danish from the Neolithic Period. structure was used for venerating Singh and has a large sextant was the first observatory to refracting telescope in British, French-American, and astronomer Tycho the Sun, the Moon, and Venus. and a meridional chamber. be located on a mountain. the world. American observatories Brahe. 92 GLOSSARY UNIVERSE 93 Glossary

interactions of galaxies, stars, planets, moons, formed by material that falls into the central a meteorite on the surface of a natural satellite Annihilation comets, asteroids, and other celestial bodies. region of the galaxy. Its mass can be a billion or a planet. Element General Relativity Total destruction of matter in a burst of energy, times that of the Sun. A basic substance of nature that cannot be Theory formulated by Albert Einstein in 1915. In as when it encounters antimatter. Atmosphere Crust diminished without losing its chemical part, it holds that gravity is a natural Carbon properties. Each element (such as hydrogen, consequence of the curvature of space-time Antigravity Layer of gas retained around a planet by its Rocky layer of the surface of a planet or natural helium, carbon, oxygen) has its own caused by the presence of a massive body. In gravity. It is also the outer layer of matter in a One of the most common elements in the satellite. characteristics. general relativity, the phenomena of classical Hypothesized force, equal to gravity and star, where the energy produced in the star's universe, produced by stars. All known life is mechanics (such as the orbit of a planet or the diametrically opposed to it. interior is emitted in the form of radiation. carbon-based. Curvature of Light Elliptical Orbit fall of an object) are caused by gravity and are represented as inertial movements within Antimatter Atom Chromosphere Distortion of light rays when passing through Orbit shaped like a flattened circle. All orbits are space-time. regions with strong gravitation. elliptical. A circle is a special form of an ellipse. Matter formed from subatomic particles with The smallest part of an element that partakes of The lowest layer of the Sun's atmosphere. It Gravitational Wave shared properties. Its electrical charge is all the element's properties. It is generally emits a pinkish-red light that can be seen only Decay Energy opposite that of normal matter. composed of three subatomic particles: the when the brighter photosphere is obscured Waves in space that travel at the speed of light neutron, the proton, and the electron. during a total eclipse. Process by which radioactive elements and The capacity to do work. and are produced by the movements of very Aperture unstable particles become stable substances. massive bodies. Aurora Circumpolar Star Also the way in which black holes eventually Event Horizon Diameter of the main mirror of a telescope or disappear. Gravity eyepiece. The larger the aperture, the more light Luminous phenomenon, with red and green Any star always visible to an observer on the The edge of a black hole. the device receives. layers, visible in the skies of the polar regions. Earth as it rotates about the celestial pole. Density Attractive force between bodies, such as The auroras are caused by the collision of solar Extraterrestrial between the Earth and the Moon. Aphelion particles with the Earth's atmosphere. Comet Degree of solidity of a body (its mass divided by its volume). Foreign to the Earth. Greenhouse Effect The point in a celestial body's orbit farthest Austral Object made of ice and rock dust. When a comet from the Sun. The Earth reaches aphelion on or approaches the Sun, the growing heat causes Eclipse Force Temperature increase caused by gases (such as about July 4, when it is 95,000,000 miles (152, Related to the Southern Hemisphere. the ice to evaporate, forming a gaseous head carbon dioxide and methane) that prevent the 600,000 km) from the Sun. and a tail of dust and gas pointing away from Visual concealment of one celestial body by Something that changes the motion or shape of surface heat of a planet from escaping into Big Bang the Sun. another. A lunar eclipse occurs when the Moon a body. space. Apogee passes into the Earth's shadow, and a solar Cosmological theory asserting that the universe Constellation eclipse takes place when the Earth passes into Galactic Filament Heliosphere The farthest position from the Earth reached by began to exist as a result of a great explosion the Moon's shadow. the Moon or any of the artificial satellites that that occurred some 14 billion years ago. Group of stars in the sky. Constellations tend to Structure formed by superclusters of galaxies The region of space around the Sun in which its orbit the planet. bear the names of mythological characters or Ecliptic stretching out through great portions of space. effects are evident. It extends some 100 Big Crunch creatures. To astronomers, the constellations Filaments are the largest structures in the astronomical units around the Sun. Asteroids demarcate regions of the sky. Imaginary line around the sky along which the universe and are separated by great voids. Cosmological theory asserting that the universe Sun moves during the year. The orbits of the Helium Minor bodies of the solar system, formed by would undergo a final, complete collapse if it Core Earth and the other planets generally lie along Galaxy rock, metal, or a mixture of both. Most asteroids were to begin to contract. the ecliptic. The second most common and second lightest orbit the Sun between the orbits of Mars and In a planet, a solid, high-pressure central mass; Collection of billions of stars, nebulae, dust, and element in the universe. It is a product of the Jupiter. Their size ranges from dozens of feet to Black Hole in a star, the central region undergoing nuclear Electrical Charge interstellar gas held together by gravity. big bang and of nuclear fusion of stars. hundreds of miles. fusion; in a galaxy, the innermost light-years. Celestial body so dense that not even light can Property of particles causing them to either Galaxy Cluster Hubble Constant Astrolabe escape its gravity. Corona attract or repel each other because of electrical forces. Electrical charges are either positive or Group of galaxies linked together by gravity. Number that measures the rate of expansion of Ancient astronomical instrument for measuring Black Hole, Stellar-Mass Upper atmosphere of the Sun. It is visible as a negative. the universe. It is expressed in kilometers per both the positions and the movements of pearly halo during a total solar eclipse. Gamma Rays second per millions of parsecs. It is currently celestial objects. Black hole produced by the explosion of a Electromagnetic Radiation estimated at 70 km/s/Mpc. massive star as a supernova. Its mass is Cosmos Form of electromagnetic radiation with Astronomy typically about 10 times that of the Sun. Radiation composed of magnetic and electric greatest energy and shortest wavelength. It is Hydrogen Another name for the universe. fields moving at the speed of light. It generated by only the most powerful Science that studies the universe. It is Black Hole, Supermassive encompasses radio waves (long wavelengths), phenomena in the universe, such as supernovae The most common and lightest element in the concerned with the physical characteristics, Crater visible light, and gamma rays (very short or the fusion of neutron stars. universe; the main component of stars and movements, distances, formation, and Black hole located at the center of a galaxy and wavelengths). galaxies. Circular depression formed by the impact of 94 GLOSSARY UNIVERSE 95

year. Equivalent to 6,000,000,000,000 miles January 4, when it is 92,000,000 miles dimensions and time acts as the fourth. Hypernova (10,000,000,000,000 km). Moon (147,500,000 km) from the Sun. Unstable Destruction of a massive star, which emits a The Earth's natural satellite is called the Moon. Spectral Analysis Tendency to change from one state into another wave of gamma rays extending great distances Lunar Mare The natural satellites of other planets are Photon less energetic one. Radioactive elements decay across the universe. commonly known as moons and have their own Study of spectral lines that provide information into more stable elements. The large, dark regions of the surface of the proper names. Elemental particle responsible for about the composition of stars or galaxies and Implosion Moon. They were originally thought to be seas, electromagnetic radiation. Photons are the most their redshifts. Vacuum but they are actually great depressions covered Nebulae common particles in the universe. Collapse of a body upon itself in response to by lava. Spectrum Space occupied by little or no matter. great external pressure. Clouds of gas and dust in space. Nebulae can be Planet Magnetic Field seen when they reflect starlight or when they The result of dispersing the electromagnetic Van Allen Belt Infrared Radiation obstruct light from sources behind them. Roughly spherical object made of rocks or gas radiation of an object so that the wavelengths The area near a magnetic body, electric orbiting a star. A planet cannot generate its of which it is composed can be seen. Radiation zone surrounding the Earth, where Heat radiation, with a wavelength between current, or changing electric field. Planets, Neutron own light but reflects the light of its parent star. Dark lines that originate from elements that the Earth's magnetic field traps solar particles. visible light and radio waves. stars, and galaxies have magnetic fields that are present and punctuate the spectrum at extend into space. Electrically neutral subatomic particle. It makes Polestar specific wavelengths reveal the composition Wavelength Intergalactic Space up part of an atom's nucleus (with the exception of the object. Magnetosphere of ordinary hydrogen). Polaris, a star that lies near the celestial north Distance between the peaks of any wave of Space between galaxies. pole. Polaris is commonly called the North Star. Speed of Light electromagnetic radiation. Radiation with a Sphere that surrounds a planet with a magnetic Neutron Star Over thousands of years, other stars will short wavelength (such as X-rays) has more Interstellar Space field strong enough to protect the planet from become the polestar. The distance traveled by light in a vacuum in energy than radiation with a longer wavelength the solar wind. Collapsed star consisting mostly of neutrons. one second (approximately 186,000 miles, or (such as radio waves). Space between the stars. Proton 300,000 km). No object can move faster than Mantle Nova the speed of light. Zenith Ionosphere Subatomic particle with positive electrical Layer that lies between the crust and the core Star that increases greatly in brightness for charge. It forms part of the nucleus of an atom. Star Point in the sky 90° above the horizon (that is, Region of the Earth's atmosphere that is of a planet. several days or weeks and then slowly fades. immediately above an observer). electrically charged and is located between 30 Most novae probably occur in binary-star Radio Galaxy Enormous sphere of gas (generally hydrogen) and 370 miles (50 and 600 km) from the Mass systems in which a white dwarf draws in matter that radiates light and heat. The Sun is a star. Zodiac Earth's surface. from its companion star. Active galaxy emitting energy as both radio Measure of the amount of matter in an object. waves and light. Most of the radio emission Star Cluster Twelve constellations through which the Sun, Kuiper Belt Nuclear Fusion originates at the core of the galaxy. the Moon, and the planets appear to move. Matter Group of stars linked together by gravity. Open Region of the solar system that is home to Nuclear reaction in which relatively light Solar Flare clusters are scattered groups of several hundred millions of frozen objects, such as comets. It The substance of a physical object, it occupies elements (such as hydrogen) form heavier stars. Globular clusters are dense spheres of stretches from the orbit of Neptune to the inner a portion of space. elements (such as helium). Nuclear fusion is the Immense explosion produced on the surface of several million old stars. limit of the Oort cloud. source of energy that makes stars shine. the Sun by the collision of two loops of the solar Meteorite magnetic field. Sunspots Light Oxygen Rocky or metallic object that strikes the Solar Mass Dark, relatively cool spots on the surface of the Electromagnetic radiation with a wavelength surface of a planet or satellite, where it can Chemical element vital to life and to the Sun. They tend to be located on either side of visible to the human eye. form a crater. expansion of the universe. Oxygen makes up 21 Standard unit of mass against which other the solar equator and are created by the solar percent of the Earth's atmosphere. objects in the universe can be compared. magnetic field. Light Pollution Milky Way The Sun has 333,000 times as much mass as Particle the Earth. Supernova Brightness of the sky originating in street The galaxy to which the Sun and the solar illumination and other artificial lighting, which system belong. It is visible as a pale band of In particle physics, a tiny, individual component Space Explosion of a massive star at the end of its life. impedes the observation of dim celestial objects. light that crosses our night sky. of matter with characteristic mass, electrical charge, and other properties. The medium through which all celestial bodies Tide Light-Year Molecule move. Perihelion The effect of the gravitational pull of one Standard astronomical measurement unit Smallest unit of a pure substance that has the Space-Time astronomical object upon the surface of equivalent to the distance traveled by light, or composition and chemical properties of the The point in a celestial body's orbit closest to the another. Ocean tides on Earth are an example. any form of electromagnetic radiation, in one substance. It is formed by one or more atoms. Sun. The Earth reaches perihelion on or about Four-dimensional conception of the universe in which length, width, and height constitute three 96 INDEX UNIVERSE 97 Index

Gacobini-Zinner, 64 Coordinated Universal Time (UTC), 75 comparison to Mars, 48 energy, 16, 34, 42 A B Halley's, 64 Copernican Revolution, 83 composition, 70 Enke division, 52 Kuiper belt, 6, 60, 64 Copernicus, Nicholas, 5, 82 continental drift, 72 equinox, autumn and spring, 74 corona Eris, accretion disc, 30, 34, 35, 37 baby universe, 15 missions to, 64 (Sun), 43 continental penetration, 70 Kuiper belt objects, 61 cosmic inflation theory, Eta Carinae Nebula, active galaxy, 34-35 Babylon, 85 parts, 64, 65 10, 11 cooling, 73 18-19, 29, 36 cosmic void, evaporation, adaptive optics system, 91 background radiation, 11, 15 types, 64 8 distance from Sun, 68 68 condensation, cosmos: universe event horizon, annular solar eclipse, 78 big bang theory, 10, 12, 13, 14, 15, 32, 34, 83 69 See eclipses, 78-79 black holes, 31 Cone Nebula, Crab Nebula (M1), evolution, Antennae (NGC 4038 and NGC 4039), Big Crunch, 14, 15 4 29 equinox and solstice, 74, 75 timeline, 72-73 constellation, crater extrasolar planet, galactic collisions, 32-33 bimonthly star map, 86 5, 84-85 essential data, 69 discovery, 61 anti-gravity, 15, 16 black dwarf (star), 23 Andromeda, 9, 85 Earth, 62 evolutionary process, 62 antimatter, 10 black hole, 4, 15, 19 Aquarius, 85, 87 Mercury, 44 formation, 13, 72-73 antiparticle, 10, 11 active galaxies, 34 Aries, 84, 86 Moon, 77 geographical coordinates, 75 creation, aphelion, 75 anti-gravity, 15, 16 Cancer, 74, 86 10-11 gravity, 69 F Aristarchus of Samus, 82 discovery, 30 Capricorn, 85, 87 big bang theory, 10, 12, 13, 14, 15, 32, 34, 83 hydrosphere, 71 filament, Aristotle, 82 formation, 22, 23, 29, 30 Cassiopeia, 85 cosmic inflation, 10, 11 internal structure, 70-71 9, 12, 29 flat perspective, armillary sphere, 82, 90 gravitational force, 30, 31 celestial sphere, 86-87 time and temperature, 10-13 lithosphere, 71 89 critical mass, flat universe, asteroid (minor planet), 5, 63 Milky Way, 37 Centaur, 85 14, 15 magnetic field, 69, 70 14, 16 curvature of space-time, forces of nature, asteroid belt, 40, 41, 63 temperature, 35 cultural interpretations, 85 16, 31 magnetic inversion, 54 16-17 astrolabe, 82 blazar (galaxy), 35 discovery, 84 moon and tides, 68, 76-77 electromagnetism, 11, 16, 17, 69 astrology, zodiac, 84-85 blueshift, Doppler effect, 21 flat perspective, 89 movements, 74-75 gravity, 11, 14, 16, 69, 76 astronomy, 40, 80-91 Butterfly Nebula (M2-9), 26 Gemini, 74, 86 night sky, 84-85, 88-89 nuclear interactions, 11, 16, 17 big bang theory, 10, 12, 13, 14, 15, 32, 34, 83 Leo, 84, 86 D nutation, 74 unified, 11, 16 fossil, geocentric model, 82 Libra, 85, 87 oceanic penetration, 70 73 dark energy, frost, heliocentric model, 83 locating, 86, 88-89 13, 14 oceans, 68, 73 69 dark matter, Roman Catholic Church opposition, 82, 83 C measuring distances, 86 4, 6-7, 11, 12, 13 one day, 72 declination, See also space exploration; stargazing motion, 88 87 orbit, 75 dew, atmosphere calendar, 5, 74, 82 mystery of Giza, 85 69 origin, 8, 73 dinosaur, Earth, 46, 66, 68, 70, 71 carbon, 12, 24 mythological characters, 85 mass extinction, 62 ozone layer, 71 G Doppler effect, Jupiter, 50 Cassini division, rings of Saturn, 52 naming, 84 21, 32 perihelion, 74 double planet, Gacobini-Zinner comet, lunar eclipse, 79 Cat's Eye Nebula (NGC 6542), 26-27 near equator, 84 58 revolution, 68, 74 64 dwarf planet, galactic cluster, Mars, 49 celestial cartography, 86-87 number, 84 5, 57, 58, 60, 61 rotation, 21, 68, 74 33 Pluto galaxy, Mercury, 45 See also star chart observing, 84, 88-89 See also solar radiation, 71 8, 9 Neptune, 57 celestial equator, 86, 87 Ophiuchus, 85 thermosphere, 71 active, 34-35 Pluto, 59 celestial sphere, 86, 87 origin, 84 tides, 76 anatomy, 32-33 Saturn, 53 Ceres (asteroid), 63 Orion, 85, 88 time zones, 75 classification, 33, 35 solar, 42 Cerro Paranal Observatory: See Paranal Perseus, 85 E water, 68-69, 71, 73 clusters, 33 earthquake, thickness, 71 Astronomical Observatory Pisces, 84, 86 72 collision, 19, 32-33 E=mc2 eclipse, Uranus, 54, 55 Chandrasekhar limit, stellar collapse, 26 Sagittarius, 37, 85, 87 (equation), 16 78-79 energy emission, 34 Earth, Venus, 46 Charon (moon of Pluto), 58 Scorpius, 85, 87 39, 41, 66-75 lunar, 79 expansion, 8, 83 atom, 10, 11, 12, 16, 17, 31 chromosphere, 43 sky changes, 84 aerial view, 66-67 observation from Earth, 79 formation, 12, 32, 33, 35 attraction, forces of nature, 16, 17, 31 clock, 74, 82 Taurus, 74, 85, 86 aphelion, 75 solar, 78 galactic clusters, 33 Einstein, Albert, closed universe, 14 Ursa Major, 85 atmosphere, 46, 66, 68, 70, 71 16 Mice, The, 32 electromagnetism, color, stars, 20-21 Virgo, 85, 87 axis inclination, 69, 74, 75 11, 16, 17, 69 Milky Way, 5, 8, 32, 33, 36-37 electron, comet, 5, 64-65, 89 zodiac, 84-85, 86-87 center of universe, 4, 5, 82 10, 11, 12 number, 9 continental drift, elliptical galaxy, formation, 65 72-73 chronology, 73 33 shape, 12, 32 98 INDEX UNIVERSE 99

stabilization, 35 Hercules Cluster (NGC 6205), 33 life, existence of, 5, 8, 12, 68, 72-73, 83 open universe, 15 Earth; Mars subclassification, 33 Hertzsprung-Russell diagram (H-R diagram), light N orbit, 40 planetary nebula, 1, 22, 23, 25, 26 superclusters, 9, 19 20, 24, 25 bending, 17 Earth, 75 planetesimal, 41 total number, 9 Homo sapiens, first appearance, 13, 72, 73 spectral analysis, 20, 21 neap tide, 76 Eris, 60-61 planisphere (star wheel), 86, 88, 89 types, 32 Hourglass Nebula (MYCN 18), 27 light-year, 8, 20 nebula, 22 Mars, 48 plasma, 43 Galileo Galilei, 5, 36, 51, 83 Hubble, Edwin, 15, 32, 33, 83 longitude, 75, 87 Butterfly, 26 Mercury, 44, 45 plate tectonics, Earth, 73 Gamow, George, 15 Hubble Space Telescope, 55, 58 luminosity, stars, 20-21, 24, 25 Cat's Eye, 26-27 Neptune, 60 Pleiades, The (star formation), 21 gas, 12, 34, 37 hydrogen, 11, 12, 17, 20, 21, 22, 24, 26, 42, 57 lunar eclipse, 79 Cone, 4 Pluto, 58, 59, 60-61 Pluto, 5, 58-59 general theory of relativity, 16, 31 Crab, 29 Saturn, 60 diameter, 60, 61 geocentric model (astronomical theory), 82 Eta Carinae, 18-19, 29, 36 Uranus, 60 orbit, 58, 59, 60-61 geology, 73 Helix, 27 outer planet, 40 polar star map, 86 Gliese (star), extrasolar planet, 61 I M Hourglass, 27 ozone layer, 68, 71 Pope, Alexander, 55 globular cluster, 21 NGC 6751, 25 positron, 11, 42 gluon, 10, 11, 17 Ida (asteroid), 63 macrospicule, 43 planetary, 3, 22, 23, 25, 26 precipitation, 69 graviton, 10 impact crater, 44, 62, 77 magnetic field, 37, 45, 51, 54, 69 solar, 72 protogalaxy 12 gravity (gravitation), 11, 12, 14, 16, 17 inner planet, 41 magnetic inversion, 54 Spirograph, 26 P proton, 11, 12, 17, 42 black holes, 30, 31 interferometry, 91 magnetosphere, 51 Neptune, 40, 56-57 protostar, 22 dark matter, 7 International Astronomical Union, 57, 60 main sequence, stellar life cycle, 20 missions to, 56, 83 parallax, 21 Ptolemy, Claudius, 4, 5, 82 Earth, 69 intrinsic luminosity, stars, 20 map, celestial cartography, 86-87 orbit, 60-61 parallels, geographical coordinates, 75 pulsar, 19, 29, 31 galaxy formation, 12, 34, 35 iron meteorite, 62 Mars, 39, 41, 48-49 neutrino, 11, 17, 42 Paranal Astronomical Observatory, 90, 91 Jupiter and asteroid belt, 41 irregular galaxy, 33 exploration, 48-49 neutron, 11, 17, 42 See also Very Large Telescope Milky Way, 37 Olympus Mons, 38-39, 49 neutron star, 22, 23, 29, 31 parsec, 20 moon, 76 mass, 16, 17 Newton, Isaac, 16, 17, 83 partial eclipse Q Newton's theory, 16, 17 matter, 10, 11, 12, 13, 30 NGC 6751 (nebula), 25 lunar, 79 precipitation, 69 J Mercury, 41, 44-45 1987a supernova, 28 solar, 78 Quaoar, Kuiper belt objects, 60, 61 Sun, 40, 41 mesosiderite (meteorite), 62 nuclear interaction, 11, 16, 17 penumbra, 43, 78, 79 quark, 10, 11, 17 tides, 76 jet lag, 75 meteorite, 62-63 penumbral lunar eclipse, 79 quasar, 19, 34, 35 weight, 69 Jupiter, 41, 50-51 See also asteroid Penzias, Arno, 15 Great Dark Spot (Neptune), 57 missions to, 51, 83 methane, 58 perihelion, 74 Great Red Spot (Jupiter), 50, 51 Mice, The (NGC 4676), galactic collision, 32 O phase greenhouse effect, Venus, 46 Milky Way, 5, 8, 32, 33, 35, 36-37 Moon, 77 R Greenwich meridian, 75 moon observation: See stargazing Venus, 47 Guth, Alan, 11 K Earth: See Moon observatory photon, 10, 12, 42 radiation, 10, 11, 12, 29 Galilean, 51 Caracol, 90 photosphere, Sun, 43 background, 11, 15 Kant, Immanuel, 32 Jupiter, 51 Jaipur, 90-91 planet, 5, 40-41 solar, 41, 67, 71, 76 Kepler, Johannes, 40, 82, 83 Mars, 48 Lick, 90 extrasolar discovery, 61 radio galaxy, 35 H Kirkwood gap, asteroid belt, 63 Neptune, 56 Mauna Kea, 90 formation, 41, 62 rain, 69 Kuiper belt, 5, 59, 60-61, 64 Pluto, 58 Paranal, 90 inner, 41 red giant (star), 23, 24, 25, 31 hail, 69 Saturn, 52 Stonehenge, 80-81, 90-91 irregular movement, 83 red planet: See Mars Halley, Edward, 33 Uranus, 55 Yerkes, 90 observation, 89 red supergiant (star), 23, 24 Halley's comet, 64 Moon (Earth's), 68, 76-77, 89 ocean, creation, 73 orbits, 40 redshift, Doppler effect, 21, 32 Hawking, Stephen, 5, 14 L eclipses, 79 Olympus Mons (Mars), 38-39, 49 outer, 40 relativity, general theory of, 16, 31 heliocentric model (astronomical theory), 83 landscape, 77, 83 Omega Centauri (star cluster), 21 remnant materials, 62 repulsion, forces of nature, 17 helium, 11, 12, 17, 20, 22, 24, 42, 57 Large Magellanic Cloud (galaxy), 28, 36 looking at, 88 Oort cloud (region of Solar System), 60, 64 revolution, 40 rift, 72 Helix Nebula (NGC 7293), 27 latitude, 75, 87 mythology, constellations, 85 open cluster, 21 See also under specific name, for example right ascension, 87 100 INDEX UNIVERSE 101

ring system solar wind, 43, 49, 51, 69 planetary nebulae, 26 shooting stars, 89 Galileo, 5, 36, 83 transparent, 12-13 Jupiter, 51 solstice, summer and winter, 74, 75 principal, within 100 light-years from Sun, supplies, 88 Hubble Space Telescope, 55 Uranus, 40, 54-55 Neptune, 56 Sombrero Galaxy, 32-33 20-21 telescope, 88 object position, 17 missions to, 55, 83 Saturn, 52 Sounds of Earth (Voyager recording), 83 red giant, 20, 23, 24, 25, 31 unaided eye, 88 parts, 88-89 orbit, 60-61 Uranus, 55 space exploration, 44, 47, 48, 49, 55, 56, 64, red supergiant, 20, 23, 24 Venus, 89 Very Large Telescope, 90-91 76, 83 reference points, 84 Very Large Telescope, 90-91 visible matter, 5, 12, 20, 30, 34, 52, 56 space-time, 16 shooting, 62 stellar map, 5, 86-87 temperature spatial dimension, 14 size, 22, 24, 26 stellar wind, 25 black holes, 35 V S spectral class, stars, 20 stellar movement, 87 Stonehenge (United Kingdom), 80-81, 90-91 stars, 20, 24 spectrum, 21 stellar remnant, 29 stony meteorite, 62 temporal dimension, 14 Valles Marineris canyon, Mars, 49 Sagittarius A and B (gas clouds), 37 spicule, 43 study, 80 strong nuclear force (strong nuclear Termination Shock (solar system region), 83 Van Allen belts, Earth, 69 Saros period, eclipses, 79 spiral galaxy, 32, 33 supergiant, 24, 28, 31 interaction), 11, 16, 17 thermonuclear fusion, 42 Venus, 41, 46-47, 89 satellite, 5, 55, 56, 59 Spirograph Nebula (IC 418), 26 supernovae, 28-29 subatomic particle, 10 tide, 51, 76 Very Large Telescope (VLT), 81, 90-91 See also moon spring tide, 76 superstition and ritual, 80 subduction, 72 time, 16, 17 auxiliary telescope, 90 Saturn, 40, 52-53 star, 84-85 temperature, 20, 24 Sun (Earth's), 22, 42-43 curvature of space-time, 16, 31 dome, 90 missions to, 83 black dwarf, 23, 24 white dwarf, 23, 24, 25, 26-27, 31 ancient peoples, 38 Greenwich median, 75 optics, 90, 91 orbit, 60-61 blue, 20 See also Sun astronomical theories, 82, 83 measurement, 74, 82 photograph, 90-91 Scorpius (region of space), 20-21 brightness, 20 star chart convective zone, 42 zone, 75 volcanism, 41, 47, 49, 77 Sedna, diameter, 61 calendar, 5, 82 celestial sphere, 86 core, 43 total eclipse self-generated universe, 15 celestial sphere, 86-87 equatorial, 87 corona, 43 lunar, 79 Shakespeare, William, 55 chart: See star chart measuring distances, 86 distance to earth, 78 solar, 78 shooting star, 62, 89 color, 20 observer's latitude, 87 eclipses, 78-79 W snow, 69 composition, 12, 20, 21, 29 polar, 87 essential data, 42 solar eclipse, 78-79 constellations, 37, 74, 84-85, 88 reading a map of the sky, 87 formation, 13, 72 water, 44, 46, 48, 61, 68, 71 solar flare, 43 convection cells, 24 stellar movements, 87 future, 25 U weak, light particle (WIMP), 17 solar nebula, 72 core, 24, 28, 31 stellar north pole, 86 gravitational pull, 31, 40, 41 weak nuclear force (weak nuclear solar prominence, 43 death, 24-25, 28, 29 stellar south pole, 87 luminosity, 20 umbra, 43, 78 interaction), 11, 16, 17 solar radiation, 41, 67, 71, 76 distance from Earth, 20, 21 types, 87 photosphere, 43 universal gravitation, theory of, 16, 17 weight, relation to gravity, 69 Solar System, 5, 38-65 Doppler effect, 21 star wheel (planisphere), 86, 88, 89 radiation, 41, 67, 71, 76 universe white dwarf (star), 23, 25, 26-27, 31 components, 39, 40-41 dust grains, 25 stargazing, 88-89 radiative zone, 42 background radiation, 11, 15 Wilson, Robert, 15 Earth, 39, 41, 66-67 end, 29 auxiliary telescope, 90 size, 31, 78 composition, 8-9, 14 wind, 51, 53 Eris, 61 final darkness, 30-31 basics, 88 surface and atmosphere, 43 cooling, 12 WMAP (Wilkinson Microwave Anisotropy formation, 13 formation, 12, 22-23, 28, 34 binoculars, 88 sunspot, 43, 83 creation, 10-11 Probe) project, 11 inner planets, 41 gas shells, 26-27 comets, 89 supercluster (galaxy), 9, 19 curvature of space-time, 16 wormhole, 31 Jupiter, 40, 41, 50-51 groups, 5, 21 flat perspective, 89 supergiant (star), 28, 31 defined, 6-7, 18-19 Mars, 39, 41, 48-49 Hertzsprung-Russell diagram, 20, 24, 25 history, 82-83 supernova, 12, 19, 22, 23, 26, 28-29 expansion, 7, 8, 10, 11, 13, 16, 32, 83 Mercury, 41, 44-45 hot spots, 25 instruments for, 82 expansion theories, 14-15 Neptune, 40, 56-57 life cycle, 22-23, 24-25, 26, 28, 30 measuring direction, 89 forces, 11, 16-17 X-Z origin, 60 luminosity, 20-21, 24, 25 moon, 88, 89 future, 14 outer planets, 40 main sequence, 20 motion of constellations, 88 T geocentric model, 82 X-ray, black holes, 30 Pluto, 5, 58-59 maximum mass, 26 night sky, 88 heliocentric model, 83 zero time, 10 Saturn, 40, 52-53 navigation, 5 observable objects, 88 tectonic plate, 72, 73 history, 13 zodiac, constellations, 84-85 “Termination Shock” region, 83 near, 8 observatories, 80-81, 90-91 telescope, 4, 82 observation, 80-81, 90-91 Uranus, 40, 54-55 origin, 22-23 recognizing objects, 88, 89 auxiliary, 90 secrets, 4-5 Venus, 41, 46-47 parallax, 21 satellites, 89 early, 81 size/immensity, 8 fdf dsdfsdaf